[chrisa]

Call yourself a consultant or freelancer, and make it clear you're looking for part time clients.

I generally do either 20 hours/week (billed monthly), or per project billing (which can be as many or as few hours as you want).

As for actually finding the work, start with people you know and let them know you're looking for a client for 10-20 hours per week; if no one is hiring directly, ask for intros to anyone they might know who might be looking.

Another thing you can do is contact small to medium consultancies in your area and mention that you have 10-20 hours of time if they have any projects they want to sub-contract; that works pretty well usually.