We have just about one more month until the tomatoes go in the ground (though with the 85° temp we hit today, maybe we should bump our target date up a little). Our seedlings were started a few weeks ago and there's no way they'd comfortably make it four more weeks in their little six-packs. So, today was re-potting day. I upgraded the largest of the seedlings to slightly bigger containers using the Future House Farm Surefire Method for Wicked Hardy Tomato Plants.*
First, I made myself an enormous omelet.

Kidding! I actually took a bunch of eggshells we'd been saving and smashed them in to wee little bits.

And mixed them into a bucket of potting soil.

The eggshells are because tomatoes need to have a lot of calcium available so that they can regulate their water intake, or something like that. Last year our tomatoes and our peppers seemed to like having some eggshells, so we figured we'd try that trick again this year.
The real key thing to re-potting tomatoes, though, is to bury them deep. All those little hairs on their stems can develop into roots, so we pull off the first leaves towards the bottom and stick them way down deep in their new pots.

That way, only the top couple of leaves will stick out and the buried stem will grow new roots.

Tomatoes are really affected by water fluctuations, so both the extra calcium and the really deep roots help a lot. Every time we replant the tomatoes, whether in new pots or out in the garden, we bury them at least halfway. They always look a bit pathetic afterwards, but being able to access so much extra water makes them grow like crazy soon after they settle into their new home.
*I just made that up. Our tomato plants were actually killed by a black walnut tree last year.