What gear do you have? As other have mentioned, many subs come with a built-in crossover which splits the signal. One part goes from the crossover to the sub, and the other part is typically sent to a set of outputs so that you can link the subs to the other speakers.
If this isn't the case, have another look at your mixer. Chances are that you have a set of aux sends; typically labeled monitor or FX with a mono jack output. I've got a similar setup with monitors running on the console main and sub connected to the FX send. The FX-send is post-fader, so that any volume changes on the fader also changes the volume on the sub. Any aux send will work, but if the mixer doesn't have any post-fader auxes you might have to control the sub volume using the aux send or aux master knob.