The first thing I'd check is the condition of the oil seal on the selector rod. If like most it's weeping, now is the time to change it. They are only cheap to replace.
Give the car a good run to get the oil nice and hot, this'll help with draining. Remove the filler plug first (NOT the large plastic plug on top of the bellhousing!) as this will help vent the gearbox when you take out the drain plug. With the oil hot and the box vented, the oil should drain easily. Obvious I know, but be careful of hot oil!
Hope this helps,
Cheers,
Richard.