1. Wait until late at night. At 9pm, your neighbors may wish to use the elevator. They will assemble into an impatient mob near the elevator door.
2. If you have stopped the elevator for your activities, don't start it again until you are fully clothed.
3. If you have disregarded the previous suggestions because you are an exhibitionist and want others to catch you, may I suggest that instead you have sex in the stairwell? Even more thrilling risk of discovery, with less inconvenience for your elevator-using neighbors.
4. "Out of order. Maintenance has been notified." No one will suspect a thing.
5. Beware glass elevators. No need to end up on the evening news.
6. If you can get a fireman's key, run the elevator up or down right at the moment of bliss. The physical rush might be awesome (rollercoaster scene in fear, anyone)
7. Make sure that you are not going to knock the emergency phone off the hook. That's trouble waiting to happen
8. Try not to make much noise. You are in a huge hollow shaft, after all.
9. Stop the elevator between floors or in the basement parking lot.
10. Use the corner wisely. Same rules as sex in an airplane go here. Use the space you have, and beware the floor.
11. Oh yeah, and watch out for the elevators with carpeting on the walls (for that warm effect). I'd imagine that stuff wasn't meant for the duty load it's getting. Rug burn on your back (or your partner's) in the middle of the day is hard to explain, in any circumstance...