[iamben]

Do this. Look for clients that regularly run banner ads or similar (or offer services / run ads yourself based on that offering). Offer X amount of time, or up to X ads per month - whatever is fair for your price. Some clients will use it all, some clients are just happy to have a designer on hand who can whip up what's needed (so money for very little).

Similarly, just offer clients retainer work. X amount per month, but you'll jump for them when needed. Example: we just had to get a designer to put something together for a really quick project. They were literally on call, turned it round in about 6 hours. Some months that's how it goes - a lot of work for not much. But other months they may not get a call.

[mpfundstein]

There was lastly some guy or girl at HN who talked about his or her company which does precisely what you say as a service! Don’t know name anymore

[mtgentry]

I think Design Pickle does this?