Went to have bloods done today.
You grab a number from the tray, and they call out the number for the next person, place it in the "used" tray and then use this number +1 for the next person to be called.
Simple, it works.
What some A*Hole did today was grab 2 numbers then stuck the lower one in the "Used" tray so the next number that was called was "his" number, meanwhile 20+ of us waiting patiently got skipped over.