True, heat mats make a difference. Even then, sometimes peppers are really stubborn. Mine took FOREVER this year, even on a heat mat. I got scared, and took seeds to Badseed to germinate for me, (that woman could germinate a fence post) so I knew for sure I would have plants. After mine finally germinated they took off, and I also got a tray of plants from badseed, so I'm flush with peppers this year. :)
I seem to remember some folks setting trays on top of the fridge for bottom heat. It seems like peppers do really well with bottom heat when they germinate (most years) ;)