I used to do what trach says and "count up" from the amount as I
am impossible at math as well.
I pulled it out of the register counting it to myself, and then I would
count it up again into the customer's hand to double-check.
Leaving the money on the counter or on top of the register like Della
suggests is a great idea too--
I started doing that when I had several short amounts so you aren't alone.