Some of the larger garden varieties (e.g. beefsteak, big boy, etc.) contain a lot of water. If this is what you're using, try Roma tomatoes instead. They have more flesh, less water.
Are you seeding your tomatoes before chopping? This removes a lot of the water. I save the seeds and pulp and run that through my tomato strainer.
I seed and roast about 2/3 of the tomatoes, mash with a potato masher, and then seed and skin fresh tomatoes to add. Roasting the tomatoes adds another layer of flavor and makes removing the skins super easy (just pluck them off).
Don't put a lid on the pan while simmering the salsa. If you put a lid on, the water that condenses on the lid adds more liquid to the pan.