I've always been a fan of Gayot.com but that will only help in the larger metros. They do professional reviews and have found them to be pretty spot on. I've found anything that's ranked 14-15/20 are very good restaurants and 13/20 for more budget spots usually means very good. They have good writeups and you can search by area, zip code, price, cuisine, etc. I also like that in each city they have a "hot restaurants" with 10 of the more popular or new restaurants in the city.

I always use a few sources though. Yelp, Gayot, and the restaurant web page itself. Yelp reviews you have to take with a grain of salt, but I like browsing pictures to get a good feel for the atmosphere inside and dishes.