Don t send originals. Figure out a way to re-create your portfolio inexpensively that you can send out. If you're a well-rounded artist, you should showcase a little bit of everything. If so, then you should show it. One good idea is to come up with a character or a concept and show all the stages of creation.
Some artists even go a step further with full sized, mechanized creations.
A sample sheet is usually a color Xerox with a number of pictures scaled down on one page. Make these look nice and slick. Your name and contact number should be on every page in case they get separated. Do your hard work justice by photographing your samples with some style and reproducing them on good photocopy machines. When sending out submissions, you have to think of yourself as an actor.