I've done a fair bit of freelancing around studio's in London, and for the last few years I've also had some responsibility for hiring and directing other freelance designers and animators.  Here's a few things I personally think I've learnt, aimed at any newcomers to freelancing.  If you're after tips specifically about motion design freelancing, check this page out for some good advice.

1. Look after your body

Okay, it's a little obvious; but your No.1 asset is your physical (and mental) health.  Without it you won't be doing very much of anything, and the effects of your day-to-day routines will last long after you've retired.  If you spend a serious amount of time working on a computer in your own home or studio, invest in a good chair that will minimise the harm to your back from all that sitting.  It doesn't have to cost a fortune -  I picked up a HAG chair on ebay for about £100.  If you don't already use a Wacom tablet and you spend most of your day using a mouse, you might want to consider switching to a graphics tablet to prevent RSI or carpal tunnel syndrome.  Drink water throughout the day to help your concentration, especially in hot or air-conditioned offices.  If you drink enough then you'll be needing to get out of your seat more often too, which is something health advisors recommend for the desk-bound anyway.  Having a hobby that is different to your freelance work, something more physical or based outdoors like DIY or sports, can really help thwart the VDU blues.  If you're the workaholic type, maybe try to include some more bodily activity into your work like life-drawing, model-making or camerawork.

2. Network in the real world

It's surprising how many successful freelancers barely have an online presence at all - who they are and what they do is already well known to many potential clients through their existing clients and their presence in the industry.  Meeting people in person is almost always more beneficial than emailing and phoning for new work opportunities.  If you don't feel too confident networking in the pub or cafe consider doing a good course in something like public speaking or assertiveness which might actually give you a few tools to improve things.  In London there are tons of ways to meet people in the creative industries.  My own line of work has regular events like Promax, Glug and See No Evil which anyone can attend, not to mention less frequent festivals and conferences.   If you feel confident enough, it can be very productive to ask someone in a senior position to simply critique your work.  You need to go into something like that with the right attitude as it's not strictly an opportunity to network and you don't want to come across as desperate or naive, but if handled well it can help your confidence and possibly lead on to other things... after all, recommendations are statistically THE most successful route to getting the best new business. 

3. Become an employer

You learn a hell of a lot about your clients' point of view simply by being in their shoes for a week.  Look out for an opportunity to sub-contract another freelancer in your line of work and then advertise online for applicants and go through the whole process of being hiring someone like yourself.  Although you may think you're putting yourself out of a job, it will open your eyes to what the people that you submit your work to are going through when you respond to a job posting from them, and you'll gain valuable insights into what it's like to commission creative services.  Wading through literally hundreds of candidates' portfolios takes a long time, and you will soon see who stands out and why.

4. Track your time

Klok, Toggl, SlimTimer, OfficeTime... There is loads of free software to keep track of your hours spent being productive.  Of course it'll help you if you have to clarify your hours to your employer, but more importantly you'll soon build up a very accurate impression of how much you are actually working, and how long different types of tasks take you to complete.  This in turn will enable you to make more accurate quotes and manage your time (and other people's time) better.

5. Get on the cloud

If you're moving around, operating on different companies machines or switching between your own laptop, desktop and smartphone, sooner or later you'll want to get a few things synchronised.  Microsoft Exchange Server is now offered as a part of some personal webhosting packages, but it's possible to get much of the same functionality with free services.  Use IMAP email instead of POP and set it up on your smartphone too so that any drafts or sent messages exist on all your devices rather than just one.  Sync your calendars by subscribing to a Google Calendar in iCal or Outlook and on your phone.  Contacts on just about any device or service can be synced with Plaxo. For essential and often-used files I swear by Dropbox, which also functions as an FTP replacement (and secure, if you password-protect files in a zip folder). I use the excellent desktop application Notational Velocity, which syncs with the free iPhone app Simplenote so that I can automatically sync any notes, lists, ideas, reminders, blog posts, scripts, recipes or whatever.  With Apple's iCloud on the horizon, a lot of this stuff looks to be made even simpler for the Mac-centric.

6. Get a good accountant

A good accountant will effectively pay for themselves.  It took me many years of doing my own accounts before I evolved a system that was truly optimal, where an accountant could have put me on the right track much sooner.  Taxation and VAT laws change more often than you'd think and if you don't keep up you could be overpaying, or underpaying and possibly incurring a fine in the future.  My accountant probably saves me at least three days a year doing tedious book-keeping - three days that I can be working on something I love.  Be aware that not all accountants are the same: some of them will merely file your books properly, others might actively recommend 'creative' ways you could pay less tax.  Try to find one that has some experience with sole traders and media-types and is a 'taxation advisor' or similar.

7. Balance your spending

When a big payment goes into your account it's tempting to go straight out and buy a new Mac / Bicycle / Wardrobe / Holiday.  Maybe you do need or deserve those things.  But I think you'll fare better during the first year or two if you remain as frugal as you can, and get a clearer picture of your average earnings throughout the year.  Then you can begin to put together a monthly budget so you can set money aside for those things and also be better prepared if you have a dry patch.  A general guide is to put aside 30% of the amount you get paid for tax and expenses.  I use a system of two bank accounts, the 'work' one gets all the invoices paid into it and pays for all my business expenses. I then pay myself a monthly amount (drawings) into my 'personal' account.  Neither of them are technically business bank accounts so the costs are minimal, but they are both with the same bank (Co-op and Smile) so when I transfer money between them online, it happens instantly.  It's worked really well for me, and speeds up my book-keeping no end too.  

8. Keep a database of contacts

Experienced freelancers sometimes end up with just a handful of clients who provide them with work year after year.  But when you're starting out I think it's always a good idea to be looking for new opportunities to excel yourself and to help others.  It can seem like there are an endless number of potential clients and it soon gets hard to remember who's who.  So keep a database that details all the contact you've had with each potential client so that you can stay in touch with the frequency and in the manner you choose.  At the very least you need to know who it was you spoke to and a record of when it was.  But it's little things like what they said about where they used to work or who their boyfriend is or something like that which can lead to a better connection between you.

9. Be discreet

Design is a mercurial and difficult-to-quantify commodity.  The reasons why one designer or studio is chosen over another have a lot more to do with personality than you might imagine, as Tip No.3 will soon demonstrate.  Whilst I doubt that it's possible to successfully change your entire personality, you can at least try to not sabotage your chances of clicking with people.  If you slag people off, gossip, lie about yourself, solicit other clients on paid time or show up with a hangover then you'll be damaging your chances of being hired again.  Some people seem to think freelancing somehow goes hand in hand with being 'rock'n'roll'.  I'm inclined to think that the opposite is actually true because you come into contact with so many different people that well-rounded diplomacy is likely to fare better than extroversion. It's probably best to simply be friendly, positive, fairly modest and 'nice'.  Then, when you've left the job behind your work can really shine for itself.  I'd also say that whilst it's good to build relationships, be wary of revealing too much about your private life, even if your colleagues do - it's just not very professional in the long run.