I would like to try to overwinter some tomato plants which I planted in a container and have worked well. I will have a hard winter and thus I need to stock them inside the house.
I think it is worth the effort, maybe if only for show and tell, but also so you can judge for yourself whether or not you think it's worth it. It is kind of neat to have a fully productive, mature plant first thing in spring. Now to answer the questions.
You will need a substantial amount of space for each plant; don't crowd them. They can still get fungal infections indoors, and very easily at that. You don't want the humidity to be constantly over 70%, but then the plants will be stressed if it stays under 30%. Somewhere in the middle is fine. You will need a very good lighting system, unless you have lots of large south/west facing windows, with good sun exposure.
Cut the plants back before overwintering them. This makes them more manageable, and also reduces the surface area of the leaves, so the roots won't have to support as much through the stressful transition. I take them back about 2/3's. Make sure it is a healthy, green leaf you cut back to. Cut out any weaker, thinner stems, to improve circulation. You want only good, sturdy, green growth on the plant.
You can expect new growth to show within a week, from the bases of the leaves. The goal is to get the plants through the winter as fast as possible, and with as little growth as possible. The growth indoors will be inferior to what forms outside, so try to make the winter as short as possible.
If you don't get a substantial amount of direct sun indoors, you could go with some bright cfl bulbs - the brighter the better; you won't get too bright for tomatoes. As for temperature, this is a temporary position, and you want the plants to grow as little as possible. Therefore 55°-60° F. will be the best, preferable the temperature will not be above 70° Fahrenheit.
Make sure the plants aren't located on or near a heating vent. Also, if you have a gas range in your kitchen, don't put tomato plants in there. They are sensitive to fumes. I knew someone who did just that (brightest place in his house) and the results were dead tomato plants in less than a month.
It's a fun project, and good for testing your 'green thumb' abilities. :) In the spring, move outside as soon as possible. The same goes for these big mature plants as the younger ones. You have to harden them off, for best results. I usually started this when the daytime temperature was over 55°F outside, even if it dipped below freezing at night. They would go out in the sun for a n increasing number of hours each day, until the last frost. By then, they were hardy enough to plant/move out.
They may not fruit right away after wintering, but they will begin to produce far before others' plants will. They take a little time establishing themselves, getting used to things, and then they will produce, often better than the year before.
