I hesitated posting this until I had ridden my commuting bike a few times this week, but I had good luck getting my rear derailleur adjusted by Jerry at Proteus Bikes in College Park. I had taken it to two shops before and the gears still slid all over the place, especially on hills. (Actually at the second shop, the new one in Takoma Park, the guy told me to take it somewhere else because he couldn't see that anything was wrong with it. Not sure how long he'll be in business with that attitude.) I have had no problem riding it four days this week after having it adjusted last weekend. Apparently Jerry is some kind of genius at repairing bikes, and prefers to fix things instead of replacing them with new parts.

So anyway, try Proteus. An added bonus is that it's a woman-owned business.