Maintaining good relations with your neighbors/being considerate is the #1 thing to do, as others have indicated. Here are two additional insights from a former house occupant in a similar position.

1. Do not purchase a downward firing sub. You don't want to put too much energy into the floor, thus turning it into a huge vibrating diaphragm. An apartment floor will (should) be insulated, but it's very unlikely that a house will have this.

2. Try to "detach" your subwoofer from the floor as much as possible. Some people even put Dynamat-type material between the sub or the sub enclosure's feet and the floor. Definitely don't put the sub directly on a wooden floor, or you might get "vibration city" downstairs.

Since low bass output is supposed to be nondirectional, I've witnessed one installation where the guy suspended the sub so the driver fired toward the ceiling. I guess an analogy woud be to take a ball and bounce it on the floor, or take the same ball and bounce it off the ceiling; which will bother the downstairs neighbor more? Unfortunately, it tended to create hot spots of bass sound in the apartment. One advantage was that you couldn't hear it at all downstairs unless really loud; in fact, the midrange sound intruded before the bass did.