Yep, just had that problem. There are several techniques.
1. With the bike in neutral at idle, grab grab the brakes fully and drop it down into first while you hold the clutch lever in.
2. With the bike on the center stand, jack the rear up so the tire is off the ground. Start the bike in gear, and with the rear wheel spinning run the revs up to 4,000 or 5,000, then pull in the clutch and step on the rear brake. May take several tries.
3. Ride the bike in 2nd gear, run up the revs per above and step on the brake per above.
4. With dry carbs (so it does not start), both brakes on fully, and the bike in gear, hit the start button. The movement of the drive train by the starter may break it free.
All of the techniques may take several tries.
Here is the thread Bob was talking about:
http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1347113454k_enn