If you're working in a creative company already, make sure you don't go into competition with your employers; you'll be shown the door quicker than you can say 'anti-competitive clause'.
2. Be honest
The most critical element of freelancing is to be honest with both clients and employers. Make sure your manager knows you're setting up shop, and don't ever 'borrow' work, contacts or pitch documents from your employer. This could be seen as corporate theft.
3. Manage your time
Get in early each day and tackle your freelance inbox and admin. This means you have the evenings clear to focus on your creative work. Similarly, if you work through your lunch hours, try and prescribe certain tasks to this time, like calling suppliers and returning calls.
4. Sort your website out
Every freelancer needs a website and yours should be well stocked. In the 'About' section, list your current creative job, as this will instil a professionalism in your work.
5. Bespoke contact
The natural follow-on to this is a bespoke email address and contact details. The last thing you want is your client desperately ringing the office switchboard, so give clear contact details.
6. Out of office
Set up an out-of-office response that states when you can be contacted, when you can respond and by what means. Make sure your voicemail is set up to respond to queries in a polite and effective manner, so if there is a client emergency they won't feel neglected.
7. Tax returns
Let the Inland Revenue (UK) or national taxation body in your country know you're working on a freelance basis. If applicable, set yourself up as PAYE to avoid tax return conflicts with your day job.
8. Bring in new business
If you're asked by a potential client to take on work that conflicts with your day job, bring it in-house. If your bosses turn it down, then by all means go for it. If your bosses do bite, you may even be rewarded.
9. Multiple portfolios
Keep one portfolio for work, the other for freelance, and neglect neither. This is where working under a moniker can really help. Give your freelance brand a name that positions it away from your day job's responsibilities.
10. Stay fresh
Finally, make sure you give yourself enough down time. Nothing will get your wrists slapped by your employer faster than a noticeable drop in the quality of your work. Always be sure to make up any missing hours from your work desk.