I have planted a tomato plant from a small leaf I found inside a tomato. It started growing in a small can, then I moved it to a larger pot. It was growing great, about 2 ft tall, but started falling over so I decided it was time for a larger pot again. Well, I moved it to a 5 gal bucket last Friday, and by Monday the leaves were real droopy. A lot of the leaves are dead and crunchy. The main stem still seems strong though. It is growing indoors, and has plenty of water. I don't know what the problem is but I would really like to fix it. I am open to any suggestions. Please help.

Tomato Gardening Tips
Soil Basics for Growing Tomatoes
Humus, the Great Equalizer
Composting for Terrific Tomatoes
Soil pH for Tomato Gardening
Grow Tomato Plants Indoors in the Fall

"You may need to gradually acclimate your plants by exposing them a little at a time to sunnier conditions before planting them. While still in their containers, place them in a shady spot. You can then move them slowly out of the shade over a period of two or three days. After a few days of increased exposure to the sun, they will be ready to place in the garden.

When transplanting tomatoes into your garden, be sure to pick a sunny location, and add a generous amount of compost or well-decomposed manure to the soil. You'll want to plant your tomatoes deeply. For instance, if the stem of your tomato is six inches long, place it three inches into the ground. Wherever the stem comes in contact with the ground, roots are initiated and the vigor of the plant increases. As a guide, place the plants in the soil up to the first true leaves. Give your plants a good long drink after you transplant them."
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The above link covers the headings mentioned and more.
Excerpt was taken from the same link.

You have to be really careful when transplanting, moving it in and out of pots is a lot of stress, so if it does recoup, leave it in this pot for good. 5 gallon is the most it will ever need, and a lot of varieties only need a pot in either a 1 gallon size or one that's 18-24 inches high and 15-20 inches wide. If your growing it indoors it needs lots of light, Either with a south facing window or artificial light. Make sure the soil stays damp but not soaked. If your plant is not getting enough light it will get tall and spindly and the tops won't hold up the weight of the leaves, which is what it sounds like was happening to your. Stake it up and help it with its weight. Hopefully you put in some organic compost around the base of the plant, you need at least 2-3 inches, it really helps the plant. When you first transplant a tomato you need to have given it a lot of warm water within 10 minutes after transplanting, and then water it everyday for 7-10 days after that to help it avoid total transplant shock. Hopefully it'll come back and do OK, otherwise plant some more seeds from what tomatoes you eat and start again!