I've used, very successfully, the SVS Soundpath Isolation Feet. (I lucked out and found somebody selling the good feet out of damaged boxes (my best guess, at any rate)) I don't have neighbors to worry about, but my floor is like a trampoline, and the before and after is significant. They run $50 per four pack, usually.
I've also heard of people using Industrial Vibration Pads: basically two layers of rubber with cork or some such material in between. Much less expensive, too. Keep in mind that you can't set cones on top of those... they have a PSI rating and a heavy sub on cone feet will probably destroy those.
One other unique step you can take is to do a two step process: this is something I did: get a wood cutting board like the ones from Webstaurant.com... needs to be the thick kind, fit that with cones to couple that to your floor (especially with carpeting),then put your sub WITH vibration absorption treatment on top of that. The board helps neutralize some vibration, and the cones minimize surface area capable of transmitting vibration (in theory)... and by adding the extra vibration treatment between sub and board, you really cut back on what the sub is able to do to the structure of the building.
I would estimate that I killed off about 85-90% of sympathetic vibrations being transmitted from my sub to the structure. Now, it takes strong LF material at high volume to make its way through the treatments to the floorboard. Yes, I can still vibrate the house, but it takes effort for me to do so.
Hope this helps!
Edit: Lovin's method involves changing your habits.
Valid. Treating for vibration control may allow you to maintain your listening habits but will cost... maybe as much as $100 per subwoofer, depending... cost of cutting board, cone feet, and soundath feet.
