dryer basics
1. make sure the vent is clear.
2. vacuum and brush out all of the lint, especially around sensors. (after doing 1 & 2 test it so you don't go taking apart a perfectly good dryer)
3. check the belt tension and guide wheels (look for lint in bearings and around shafts)
on gas dryers use a brass brush and clean the burner and igniter (for some reason spiders love to clog up gas lines)