My lovely tomato plants have grown their 2nd set of leaves. 14 little plants made it through my learning curve of seed starting.
I'm putting a lot of trust in Nancy Bubel & this book: The New Seed Starters Handbook
I changed a few things I was doing based on her ideas. First, I removed the heat source. Her opinion (citing several studies) is cooler temps during seedling growth, lead to higher yields. So, once the seeds germinated, I removed the heat mats.
Also, lots of websites suggest 24 hours of light. Nancy suggests 8 hours of darkness leads to better growth and I can already see the difference. (She cites several scientific sources)