It really depends on what type of heating pad and how old it is. Some come with built in thermostats which will allow you some control over the heat output (I think these are actually more like a dimmer switch). If you don't have the reptile type heating pads, human heating pads can be used, providing you do not put any weight directly onto the pad. (ie prop up the end of the tank a quarter inch and slide the heating pad into the space.
Regardless of which type of heating pad you are using, it cannot be buried in the substrate. To help prevent overheating, making sure there is air flow around the heating pad will ensure temperatures don't get too hot (it can crack glass aquariums if the tank is lying directly ontop of the pad (or the pad is glued to the bottom of the tank and both are flat on a table).
Raising the tank a quarter to half inch above the heating pad, will ensure some air flow gets underneath the tank and keep the heating pad from getting so hot it will crack the glass. This will also help prevent the heating pad itself getting too hot and shorting out, risking a fire.
Using a heat pad with a built in thermometer or at least a 'high, med, low' switch, will make it easier to regulate. I use human heating pads with three settings and without the auto shutoff feature for my snakes. I keep the thermometers on the low setting and these never seem to get hotter than 95F which is good for most of my snakes.
As you don't need your heating pad on constantly (additional heating in the daytime can be provided with another house hold bulb or a ceramic heat emitter.) you can put it on a timer so it only comes on when the lights go out and turns off again not long after lights come back on.
-----
PHLdyPayne