musts for my last birth were:
a waterproof covering for the bed (most of my birth was on my bed, that's were my water broke)
2 sets of sheets (one for during, one for after. if you don't have two sets of sheets, you'll probably want another for after baby comes anyway. can't count the number of times we've had spit-up on our sheets and not been able to wash them in time to sleep again)
chux pads (to try and catch most of the mess, both during the birth and after for...