I learned this thanks to Mel Bartholomew's All New Square Foot Gardening and thought I would share.
Bartholomew points out that tomatoes can sprout new roots along their main stem so if you lay the plant down when you plant it, "They will develop an enormous number of roots, all along that hairy stem, which will sustain a larger, more productive plant." I never knew that before, but I tried it last year for the first time and it was amazing! Here's what you do:
Then you take your little seedling, like so.
I'm afraid it looks like I'm pinching the plant, here, but I'm not. Do be careful when you bend it.