You might have already guessed this but, I love starting seeds. It kind of goes along with my
I think timing is the first big one. So many times I have started way off the mark, either too early or late (um, starting tomatoes in June anyone?). It wastes money for one thing and it just doesn't make for a real productive year! I am a big fan of an almanac (either can be found online or purchased in a small local store). It will give you approximate frost dates and planting by the moon charts if that strikes your fancy. Always read your seed packets for exact instructions but here are some references that I also deem useful:
Almanac Chart, just plug in your zip code for personal dates.
This chart needs you to plug in your first frost free date at the top.
Water is the next big thing as it can be kind of tricky. I chronically forget to water my seedlings. Sometimes this works in my favor because my seedlings never become water dependent. Or, sometimes they just die. What can I say? I'm a bit scatter brained these days! If I were to water properly I would keep seeds and very young seedlings consistently moist and then once the first true leaves appear then I would back off on the watering a bit. I try to water about 3 times a week or every other day at this point while they are in single cells. Then after I pot on anything in need I try to only water them every 2 days. The reason behind this is preventing water dependence and shallow roots. When you soak thoroughly a few times a week it then forces the roots to drive deeper to access the water being stored in the bottom of the pot. Many people also like to bottom water with a tray but I am not a huge fan of this. Personal preference, experiment with what works for you!
Potting on is the last thing to seedlings. Most plants that need to be started more than a few weeks ahead will need potting on at least once to prevent root binding and "legginess". I like to pot my tomatoes on when they are about 3 inches high and I bury them right up to their top leaves. I have found that little 4 ounce yogurt cups are the perfect size for the first potting on. Tomatoes I tend to like to pot on 2-3 times depending on when I can get them in the garden. Things like greens usually don't need much start time and I try not to waste space with potting them on (the bigger the plant to bigger the container). Also, onions do fine in the little cells until it is time to plant outdoors once I transfer them from the flat and having them in cells if better for their roots when transplanting.
Starting seeds is a skill! It takes trial and error and perseverance but it is a very worth while knowledge. Happy Spring everyone!