How to extend the freshness of salsa?
Acid is your friend here. You have some lemon, and tomatoes are acidic, but apparently that isn't enough. You should get at least a good week out of fresh salsa (mine lasts longer than that). Try adding a good shot of plain, distilled vinegar. Many recipes for salsa (including my own) include vinegar; add as much as you can without negatively affecting the flavor.

A bit of salt wouldn't hurt either, again, just to the maximum for good flavor.