Used too much lemon juice in Mediterranean salad
Might as well make this an official answer --

The easiest way to deal with too much of an acidic liquid, when there aren't any other liquids involved is to simply drain it, and see how it is.  If you don't have a suitably sized strainer or colander, you can use a slotted spoon to transfer the salad to another bowl while leaving the liquid behind.

If it's still too acidic after that, if it's in a colander or strainer you can try giving it a shake to get more liquid off ... although it might end up crushing the tomatoes.

You can also try rinsing the acid off -- you can use water, but it can mute the flavors and you may have to then drain it again and try adding back in some acid.  An easier approach is to use a lemon-lime (or other citrus) soda -- it has some acid, the citrus taste, but also some sugar to help balance out the acid.

... and failing all that, you can take Paparazzi's suggestion and add something with some fat in it -- olive oil or cheese can help to balance out the acidity.  (although we're usually working the other way -- acid to brighten up heavier fatty foods)