Heat, even in my centrally heated home, is the hardest thing to supply those developing seedlings. Don't even get me started on sunlight, which is something we just don't have in the spring. I use a seedling germination mat to maintain the even 75 degrees for good sprouting. As soon as the little guys come up I shunt them under a fluorescent light for 16 hours a day to make up for the dim conditions in a windowsill.
These gadgets help get the seeds going, but how do I keep them from becoming root bound when cold temperatures push back the planting date? Repotting is the only answer. I start in 3-inch pots, but some years I have to transplant them to a 5-incher before planting time comes. While I usually don't like to do multiple transplanting (it is stressful on roots after all), in the case of tomatoes and taking advantage of our short growing season, it's a necessity.
Minimize stress by using the same type of soil in the new pots. Transplant before they become root bound. If the roots are crawling out of the drain holes you've waited too long. Finally, only up the pot size if you have to. Starting way too early, say February, results in unnecessary repotting instead of larger plants.