Light. By keeping a grow light or other source of light just a few inches (3"-4") above the seedlings' tray, the little plants don't have to reach for light and can develop stronger stems.
When the seedlings start to grow, you can raise the lights about an inch at a time, keeping them close to the seedlings, but of course not touching.
What fertilizer are you using on your seedlings?

Thanks so much! That is a huge help

Thanks for reminding me to put in seeds!
Tally HO!

Yikes! It's already time to start tomatoe seeds?

I know I can transplant leggy tomato plants deeper but how about eggplant seedlings. I usually buy the plants but decided to start them from seed this year and they did come up leggy. I now have them just about touching the lights and they are healthy but I would like to start transplanting them into small pots and wonder if I could plant them deeper.