Here are some tips to help get the seedlings off to a great start:
Test medium-size and large seeds for viability by dropping the seeds in a container of water. Good seeds will sink to the bottom; nonviable seeds will float.
If in doubt about how deep to plant a seed, cover it lightly. If a seed is planted too deep, it will not sprout.
To speed germination of many seeds, water them the first time with a fine spray of hot water. Thereafter, water with lukewarm or room-temperature water, never with cold water.
Seedlings need the maximum possible amount of light when growing indoors (though strong, direct sunlight can be too intense). Reflective surfaces near the plants will bounce additional light back onto them; white-painted walls and sills are more reflective than dark colors.
Thin overcrowded flats or pots to make sure there is adequate growing space and good air circulation.
Do not overwater.
Source: “500 Terrfic Ideas for Gardening” by Anne Halpin