I make it a habit to draw lines through my amounts and put the $ right next to my total paid. Like '$40' not '$ 40.'

I usually go for even amounts like $48.00 and round up tips but instead of writing '$48.00,' I write '$48-------' (solid line to the end of the paper) so they cannot write anything else or change the 0 to an 8.

This works for me, no fradulent charges posted yet.