DON'T eat near a major tourist site
The food served near any major attraction is usually going to be double the price, and not nearly as tasty or authentic. The reason is simple: when restaurants know customers aren't coming back, they don't worry about quality. And what do tourists know about "authentic" local cuisine anyway, right? They're just happy to tell friends back home about the "amazing" pizza they ate in front of the Colosseum.
What to do instead: Follow the "six-block" rule. Walk six blocks in any direction from a major tourist attraction. The further away you are, the more local, cheap, and delicious the food will be. Use websites like, well, Thrillist to find the best restaurants and Foursquare or OpenRice to find out which spots the locals are rating highly. Finally, whatever you do: please, PLEASE, avoid restaurants with glossy menus in multiple languages. Just keep walking.