Just sprinkle a small amount of baking soda on the soil around your tomato plants. (be careful not to get any on the plant itself and be also careful with young seedlings which are more tender.) The baking soda absorbs into the soil and lowers the acidity levels. This will give you tomatoes that are sweet rather than the tart flavor that store bought tomatoes have.
As with all home remedies, be sure to test with just one plant when trying this. If your soil is already quite alkaline, it could affect the level too much by adding soda.)
You can also use baking soda with canned tomatoes when making sauce if you like. It will sweeten them without having to add extra sugar (and calories!)
To see more: http://thegardeningcook.com/sweet-tomatoes/