DON'T DENT THE CARDS!! --- Address the envelopes before you put the card in it/or use labels...So you don't leave pen marks on your cards
GLUE STICK!! --- Use a glue stick to seal your envelopes...don't lick 200 if them!! Or wet a washcloth & line a bunch up at once & run the washcloth along the glue.
HAND CANCELED!! --- Ask the post office if they will hand cancel your cards so that you get the pretty stamp on them rather than digital printer marks. My post office gladly hand cancelled mine this year, over 100 of them. But you have to get there early when they first open for the best chances of this.
ON THE FLIP SIDE!! --- Alway put your card in so that the front of the card faces the back of the envelope and correct side up, this way the recipient will see the front of the card first when they open it.