Be honest.
Don't lie. Don't cheat. Don't deceive. Don't hide. Bloody simple isn't it? Have respect for your partner because that's what they are; A partner. Treat them like one. No, I'm not bitter.

Be affectionate.
If you're with someone its because to you, they're special. Treat them like it. Hold them, tell them what they mean to you, take them out, give them something, do anything you can possibly think of to let them know they're not just a friend you doink.

Put out.
Just do it. And not just missionary or handjobs. Try the freaky shit they bring up half jokingly while looking intently for your reaction. Seriously. It doesn't take all that long, its a great time killer, and who knows? You may find out there is a whole other freaky world out there that you absolutely love. Worst case scenario is you have to shower again and theres an awkward silence for a couple hours as he/she struggles to apologize for the surprise penetration.