Sometimes I like to put a little paper strip on the inside...first of all it uses up paper scraps but I also think it gives a nicely "finished" look to your handmade card.
I also used a Corner Chomper by Crop-a-Dile which you can see here:
I like those Corner Chompers but I did have one break on me. The little plastic guide broke off and can't be fixed...so take care of yours if you have one! The Gadget Queen does not like when her gadgets do not hold up!