Cut the jack-o-lantern hole in the BOTTOM of the pumpkin, not the top.This serves several purposes:

1 - you can cut at an angle to help the pumpkin stay straight (if you have an odd shaped pumpkin).
2 - you can put the candle on the bottom without risking burning your fingers to light it from above.

(Via Parenthacks)

Nay, indeed, good sirs, a better solution yet awaits: cut the hole in the form of a keyhole, with the circular part around the stem as usual, and the long trapezoidal part down the back. Thus you can reach in and mess with the candle from the side, without burning your hand or moving the pumpkin; and the lid of the pumpkin will always go on in the correct fitted orientation; and there is much less chance of the lid shrinking and falling in. It also makes it easier to access the area behind the face of the pumpkin, so you can carve it thin to let the most light through.