The first is the physical part, which involves everything from different sexual acts through to different positions used in penetrative sex. The position plays the biggest part in physical pleasure, as different positions will allow you to stimulate different parts of your partner's body. If the 'cowgirl' (girl sitting on top facing partner) doesn't do it for your other half, try turning around and facing away from him - the 'reverse cowgirl'. This creates a small amount of downwards force (towards the feet) on your partner's penis, adding another dimension to his sensation, but it can also mean your G spot is more easily stimulated; it's a win-win. In this position, the girl is usually on her knees, but a variation is to crouch on your feet. This will provide more control over depth and speed - two big factors in the physical side of sex. Another thing to try is pelvic clenching exercises, which help the girl gain yet more control.
The second part is the emotional or psychological aspect. Many people are as aroused by the idea of something as they are of the actual act itself, and one thing that almost all men enjoy is a girl with a sense of adventure. Trying new sexual positions will be exciting for your partner not only because of the new physical stimulation they provide, but also because of the confidence their partner demonstrates by exploring sex with them. To this end, you may want to invest in sexual aids, which range from the very basic to the apparently terrifying, and are readily available online from places like www.lovehoney.co.uk
Above all, only do what you are both comfortable with, and always ask your partner what you can do for them to make their experience better. Everyone is different in so many ways, and there is almost nowhere where that is truer than in the bedroom.