Less than a few months? Just park it with a battery tender hooked up (CTEK is popular on most higher-end cars forums).
In 2000 I left an E46 parked outside for over six months while out of the country (no battery tender, just left at uncovered off-Airport storage). Cranked up as normal on my return.
BMW's often have battery issues these days though. Longer than three months think about tires sitting on the same patch for too long and fuel going bad. I put fogging oil in my stored lawn equipment (large-ish tiller that's rarely used etc) + sprayed various areas with WD-40 but have never done it with a car. Did it with a motorcycle left for over a year.
Driving a few miles once a month won't hurt; I try to do it if we leave a 3rd car on the lift for too long .
Six Euro Deliveries since December 1998 (Owned E46, E90, E91, E92 and E93)
28 vehicles in 26 years of driving so far. Guess I like cars
Last edited by MP3_E46; 01-01-2013 at 08:15 AM.