Do have a selection of samples ready in differing styles. For example, cards done in pencil, pen, and paint, or line art, gray-scale and full color.
Do show your work in a style that you feel you can maintain for 100+ cards. Its no good to spend 2 hours on a sample that will get you the job, and then not be able to complete the job to that standard. When I was trying out for sets, I would do sample cards that took me no more than the amount of time I knew I wanted to spend on the actual cards themselves, usually about 20 or 30 minutes.
Do limit your portfolio to your top 3-5 pieces of artwork. An editor will know in that amount of time if they want to hire you or not, and the extra 10 or 20 pieces you put in are not going to change their minds. It's just going to waste time and possibly (probably) annoy them.
Don't forget that everyone reads message boards: artists, collectors and yes, even editors. Ultimately, if you want to be a professional, act like one; even online.
Don't lie about your abilities. If a set requires an artist that can do 250 cards, and you can only do 100 in that time frame, then that set is not for you. It doesn't matter that its your favorite character, or a licensed property that would be great to be involved in. You only end up hurting your own reputation, and disappointing collectors when you commit to something you cannot actually accomplish. Again; if you want to be professional, act like one.
Don't break the rules. If your contract says "don't draw this certain thing" then don't draw it. If it says "don't do after-market cards" then don't do them. When artists feel that the rules they agree to don't apply to them, it makes things worse for the entire community. Too many times we've seen un-necessarily strict rules written into contracts because on a prior set someone wasn't able to use good judgment. Don't be the artist that makes it harder on everyone else later in the game.