I've used tomato paste to thicken up salsa but a recent salsa recipe I made had a good tip which was to let the tomatoes drain for about 30 minutes in a strainer. Came out decidedly thicker than previously given about the same mix of tomatoes.Here's the recipe. Because I was using slicing tomatoes the first batch is a little thin, so I added about 2 tablespoons of canned tomato paste to the second batch and it helped. Use freshly-ground black pepper if you have it; it makes a difference, and I generally use a little more than a 1/2 tsp.
(from USDA bulletin 539) yield: 6 to 8 pints
5 pounds tomatoes
2 pounds chile peppers
1 pound onions, chopped
1 cup vinegar (5%)*
3 tsp salt
½ tsp black pepper
Roast and peel peppers if they have tough skins (not necessary for jalapeños or serranos), remove seeds and stems, chop. Scald and peel tomatoes; chop. Combine all ingredients in large saucepan. Bring to a boil and simmer 10 minutes. Ladle into pint jars, leave 1/2 inch headspace. Adjust lids and process in boiling water bath for 15 minutes.
*Notes: I generally use jalapeños, seeds and all, and I find that ½ cup bottled lemon juice plus ½ cup white vinegar tastes better than using all vinegar (or all lemon juice.)