Leave it in drive. The less you move the moving parts, the less they wear. each time you go to neutral, you remove hydraulic pressure to a band or clutch pack. Constant back and forth can't be in any way construed as being useful. Doesn't sound like you're racing from the lights, so leave it in D and stay off the throttle until you're ready to depart the premises.
[This message has been edited by jeffsr (edited 01-17-2000).]