It's no secret that seeds sprout better indoors with a little extra heat, and an electric heating mat (like this one: ViagrowTM Seed Propagating Seedling Heat Mat, 20.5" X 8.5") is a safe and relatively inexpensive way to give your little plants a head start. But if for whatever reason you don't want to commit to buying one, there is another way.
I was a little skeptical about possible safety issues when I first tried this out, but 5 years without an issue and some poking around with a thermometer has reasssured me. If you still have qualms, just don't leave it running when you leave the house or sleep; even a few hours of extra heat a day should give your plants a little kick. My latest batch of marigolds seeds sprouted in 48 hours on 14hrs of heat a day! Of course YMMV.
The method is simple. You will need:
Just stuff the lights into the plastic strawberry container, set the cookie sheet on top and then put your containers of soil, jiffy pellets, etc atop it. Be sure you're using some sort of cover to prevent the seeds from drying out.
You must be very careful about keeping the moisture away from the lights, obviously. I found that with my lights it never got hotter than 130F inside the strawberry container among the lights, or 110F right on the surface of the metal tray. You should measure your setup to make sure your heat levels are not excessive. If the lights start to scorch or melt anything they are touching, they are way too hot; find some other lights or go get a real heating mat.
But if they chug along at a pleasantly warm temperature for hours on end, you are in business. Check inside your containers every day and use the extra heat just until you have a good germination (usually 3-7 days).