This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Marie Martinek offers:
In addition to all the recipes you'll be garnering, I want to suggest my
technique of Time-Shifting. This requires that you have freezer space....
As the tomatoes get ripe, wash them and chop them and toss them into a
gallon-sized ziploc bag. I put my bag into one of the tupperware
juice-or-cereal pourers, the tall rectangular thingies, to make it stand up.
When it's full, pull it out of the holder and put it in the deepfreeze. Do the
same thing with hot and sweet peppers (but in a pint bag).
In January, or sometime when the heat & humidity isn't in the 90's, pull out 2
gallon bags of tomatoes, and 1 pint bag each of sweet and hot peppers. Peel
them out of the bags (they're usually snagged enough by then that I don't try
to save the bag; just cut it off) and put them in a big pot. As it thaws,
scoop into a colander over another big pot. As it drains, scoop the pulp into
YET ANOTHER big pot.
Eventually, you will have one potful of pulp, and one of juice. Put the pulp
back in the fridge. Run the juice through your finest strainer, or a food
mill, to pull out some of the excess tomato seeds. Put back into big pot.
Start cooking the juice down. Once you get it to boiling, turn down to simmer
and let it go for several hours, stirring every once in a while. Let your
kitchen windows steam up.
Once the juice is reduced by at least a half, add it back to the pulp and
continue on with your recipe.
I don't even take the skins off -- my husband doesn't mind a bit of roughage
in his salsa.