Can't beat word of mouth. It's as cheap as it gets.
If you just want an "on the side" job, I'd probably start out by knocking on doors or using door hangers initially. Target the neighborhoods you want to get into. No sense taking out an ad in the paper that may or may not get you a return. If possible, take along pictures of work that you have done, so if anyone is interested you can show them what you have done.
One piece of advice- if you are only wanting to do this as a "side" job, and not wanting to get into it full time, PLEASE, for our sake (and really for yours too), price it like you were in it for real. Get yourself insurance too. If you start out lowballing, you will have that reputation for a long time. Lowballers that have no overhead because they are not in it for real
really deserve to be shot! Don't take that the wrong way though!
Good luck with it, and don't be afraid to come back here and ask ???'s.
Dan