This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Many subwoofers contain their own amplifier and crossover.
For these, take the preamp output and feed it into the subwoofer
amp input and also into the main amplifier.
For other subwoofers, just run them in parallel with your main
speakers, or combine them into your system with your own bass
amplifier and crossover.
Some A/V receivers contain a splitter specifically for use with
subwoofers. If you have one of these, you will either want a
separate amplifier for your subwoofer or an amplified subwoofer.
Consult the manual which comes with the subwoofer.