Seedlings don't have to stay as warm as germinating seeds. Move them away from radiators and air vents, or off the heating mat, as soon they have germinated.
If youre using a soilless mix without compost, begin to fertilize your seedlings as soon as they get their first true leaves. (These leaves emerge after the little, round cotyledon leaves.) Water with a half-strength solution of liquid fish/seaweed fertilizer every week or two. Use either a spray bottle or add the fertilizer to the water you set the trays in if you're using the wick-up method described above.
Give them room
If the seedlings outgrow their containers or crowd one another, repot them into larger containers filled with a mix that includes compost. Extract the seedlings with a narrow fork or flat stick, and handle by their leaves and roots to avoid damaging the fragile stems. Tuck the seedlings gently into the new pots, and water them to settle the roots.
Lightly ruffling seedlings once or twice a day with your hand or a piece of cardboard helps them to grow stocky and strong. Or, set up a small fan to gently, continuously blow on your seedlings.
Toughen them up
About 1 week before the plants are to go outside, start acclimating them to the harsh conditions of the big world. Gardeners call this hardening off. On a warm spring day move the containers to a shaded, protected place, such as a porch, for a few hours. Each day—unless the weather is horrible—gradually increase the plants exposure to sun and breeze. At the end of the week leave them out overnight; then transplant them into the garden.