I have a fool-proof way of counting up to 100 using just my two hands:
Right hand: count 1 - 5 by tapping one thumb/finger at a time on any surface (?leg?); count 6-10 by keeping thumbs and fingers down, ie:
1 = thumb
2 = index
3 = middle
4 = ring
5 = pinky
6 = thumb + pinky
7 = thumb + index
8 = thumb + index + middle
9 = thumb + index + middle + ring
10 = thumb + index + middle + ring + pinky
at which point, start counting "10s" in the left hand, using the same scheme:
11 - 20 = left thumb + one of the above in the right hand
21 - 3- = left index, + ...
100 = all fingers in both hands down, start again if necessary!