I live in Madrid (I used to live in ín the District--for 12 years). I'd be happy to help out with tips here, especially since I plan to plunder this site for up-to-date info for my next visit to DC.
The more specific you are, the better I can help. Info like where you are staying, what days you'll be here, what your plans in the city are, what kind of food you like, phobias, tolerance for smoke and noise, whether you mind eating standing up, etc.
My number one suggestion for enjoying food in Spain is to force yourself immediately into the schedule, because it is immutable (no 24-hour diners here and you'll be eating everyone else's dregs during off-hours). For some reason, our American instincts often instruct us to try to avoid crowds and do things at off-hours. This doesn't work in Spain. Here's the basic Madrid schedule. This may differ slightly in other parts of Spain, particularly the north where people tend to eat dinner somewhat earlier.
breakfast/desayuno
before 11:00
i.e. cafe con leche and toast (with olive oil!) or churros (fried in olive oil!) or pastries in cafe/bars.
morning snack/sandwich break/almuerzo
11:00ish
In cafe/bars. Some breakfast stuff still might be available, but usually not as fresh (except on the weekends). Lots of people have a "pincho" de tortilla at this time.
lunch/comida
2:00-4:30
The biggest meal of the day--most places only do one seating, with people eating for hours, so don't show up at 4:30 expecting to beat the rush.
afternoon snack/merienda
5:30-7:30
Chocolate con churros if you missed them for breakfast. Mostly for kids and old ladies (and me!).
7:30-9:30
Paseo time. Go for a walk. Have drinks. Eat potato chips (fried in olive oil!). Window shop. Think about your next meal.
dinner/cena
9:30-12:00
Some tapas bars open a bit earlier. Restaurants may open at 9:00, but don't really fill up until 10:00.
