I just got this heated mat with a wire rack that goes around it to keep the seedlings a little bit off the mat. Anyway, you can set a temperature and it maintains it thermostatically. I'm waiting to use it because it's still a bit early but I'm really excited. Anyone used one of these? Any tips?

Here is a link that might be useful: Gro-Mat

If you want bottom heat then some sort of heating applied from below is the way to go. Sound too obvious to be true! Heat mats are a simple solution. For larger areas, you can bury a heating cable in sand below your plants. Active heating like this requires a thermostat. Some cheap mats don't include a thermostat, sooner or later you will kill your plants.

Other sources of bottom heat include the top of your fridge. It is barely warm to touch but you are trying to maintain a temperature at the roots of perhaps 5F above the air temperature, not cook a chicken!

Lastly, I rarely use bottom heat. I use an enclosed propagator which is heated by the lights. The problem is nearly always providing enough ventilation to keep it cool and I have no need for additional heat.