Do's:

Be respectful. Concentrate on the ladies. Make sure to bring your A game in regards to oral and finger play. Always be touching or tasting, even if it's just a light caress while they're concentrating on each other. Pay attention to their responses. Go with their flow. Make eye contact. Respect any boundaries, whether established on the fly or beforehand. The latter is especially important if there's already established relationship(s) in the group.

Don'ts:

Don't be selfish. Your dick is not a gift. Unless something goes horribly wrong, it's gonna get action. Be patient. They will let you know in no uncertain terms when they want it. Don't favor one over the other (especially if one is the "new girl"). Don't be intimidated if they want to bring toys into the mix. Toys can do things you can't, and vice versa. Don't let pride get in the way of their pleasure.

tl;dr If you make it about them and not about you, the odds of getting an encore go way up.

Best of luck, and enjoy.