It's easy to count to 9 with a hand, and you can count all the 9s with the other hand up to 81. 5 fingers, and 4 spaces in between the fingers. I do the space by holding both fingers up at the same time. So first, first and second, second, second and third, third, third and fourth, fourth, fourth and fifth, fifth.
Another way counts to 100, but you do gotta keep track of counting to 4 each time. I do each finger on 1 hand 4 times each. So 4 times first finger, 4 times second finger, 4 times third, .... all 5 fingers 4 times each makes a finger on the other hand. So the other hand ends up each finger means 20. So when you're done, you got 100. This is easy to do when I'm driving. Just have to keep track that you are doing each one 4 times and not 3.
I know some people recommended counting sections of fingers, like there are 3 sections of each finger. But I know that would never work for me. If I tried it that way, I would forget to keep track of it and lose my place.