tomato plants are dying

If the plants are wilted without any discoloration, its bacterial wilt, the bacteria lives in the soil and there's not much you can do. I would find another spot to grow tomatoes, sometimes just a few feet can solve the problem, also be sure and don't overwater. If you destroy the plants look inside the stem, if its bacterial wilt it will be watery and turn brown.

 

One of the most important things in tomato culture is to make sure that no soil comes in contact with the leaves, a lot of tomato diseases are soil borne. When you plant your tomatoes make sure that you use a thick mulch to keep rainwater from splashing dirt up onto your leaves and do not overhead water your plants, irrigate or drip instead. Some blights can be brought by insects that suck on the juices of the tomato plants so try and control white fly and aphids. If you smoke, do not touch the leaves with an unwashed hand. Most of your better tomatoes, heirloom varieties, do not have much resistance to these blights and once infected there's not much you can do. Even some of the resistant hybrid varieties are still affected by these blights----they are resistant, not immune. The last resort is container planting.

 

I have also had trouble with all my tomatoes dying for three years now, even though I did plant in different places, so this year I only put out two in pots. I used potting soil and still they are dying. The lower leaves turn yellow with spots and then the next leaves up and so forth. Any suggestions other than never planting tomatoes??

 

Sounds like early blight, the brown spots will have concentric circles if its early blight.

Baking Soda Spray: for anthracnose, early tomato blight, leaf blight and spots, powdery mildew, and as a general fungicide Sodium bicarbonate commonly known as baking soda has been found to posses fungicidal properties. It is recommended for plants that already have powdery mildew to hose down all the infected leaves prior to treatment. This helps to dislodge as many of the spores as possibly to help you get better results. Use as a prevention or as treatment at first signs of any of the diseases.
To make: Mix 1 tablespoon baking soda, 2 1/2 tablespoons vegetable oil with one gallon of water. Shake this up very thoroughly. To this mix add 1/2 teaspoon of pure castille soap and spray. Be sure to agitate your sprayer while you work to keep the ingredients from separating. Cover upper and lower leaf surfaces and spray some on the soil. Repeat every 5-7 days as needed.