Whenever I do a convention or a signing I always have a rubber-banded batch of blank sketch cards with me so I can bust out some quick marker drawings to give to the fans and kids that I meet.
There are lots of artists who do some beautifully detailed work in sketch card format (often fetching some deservedly handsome prices) but that doesn't fit my style or personality at all. To me drawing a sketch card is like giving somebody a hot dog at a neighborhood cookout - a small gift between new friends. So I just do 'em quick and dirty and always for free (but… SHH! Don't tell anyone!)