how does everyone keep their tanks at the correct temp?

two retfs, 24x18x18 exo terra. even low wattage bulbs (40) are bringing the heat up too high and too fast (it's mounted about four inches above the tank), and are therefore not reliable to leave on while i'm at work. i just bought two 40watts - a red and a blue - and since sun has set here, tried the red to see how long it would take to reach temp. it was 73 when i turned the bulb on; within about 15 minutes it was already at 77.

i looked into timers, but it was overwhelming, and it's difficult for me to trust that it won't fail and fry them.

so far the terrarium naturally holds between 70 and 73, which isn't awful, i suppose, but once temperatures drop this winter, i'm going to have a problem; i'm looking for a way for heat to kick on when it hits 73 and turn back off when it hits 76. with such a narrow margin of error, i'm not sure what to do short of quit my job and become a full time frog caretaker.

any advice?