2. Arch your back. This is the single most potent bedroom posture a woman can assume. Most women do this naturally during heightened sexual pleasure, and he becomes more aroused by seeing you aroused. Do it regardless of being face down, face up, or on your side.
3. Point your feet down as if you were standing in high heels. I haven't figured out why this is sexy yet, but rest assured: it is. Do this when you are lying on your back with your feet in the air, crawling across the bed, or in any other situation in which your feet are free. Consider leaving your heels on during sex, as it assists and heightens effect.
4. Throw your head back. This is also done naturally during heightened sexual pleasure. Combine with #2 and #3. Also try turning your head to the side, as if the pleasure is too intense to look directly at him. In addition to demonstrating your pleasure, both of these actions slightly contort and accentuate the neck, one of the most delicate and feminine parts of a woman's body. Close your eyes or it will seem unnatural.
5. Wear matching underwear. This makes you more comparable to the supermodels he fantasizes about routinely, and generally looks a lot better on you (models do it for a reason). Simple colors and types of underwear are perfectly acceptable. Also, let him initiate the removal of your underwear. I am usually disappointed when a woman takes hers off too soon. It looks sexy and I don't get to see it very often, so I want to enjoy it for a minute before I rip it off.
6. Don't pull his upper body down towards you during missionary or similar positions (e.g. to make out or hold him while he penetrates you). This will prevent him from watching himself enter you, which is hugely erotic.
8. Make your movements slow, sexy and constant. The general idea is that you are slowly but continually moving in rhythm with and response to his sexual lead - in a sense, "flowing." The movement is something like squirming, but in a slower, more sensual (rather than uncomfortable) way. He will be turned on by watching you respond to the pleasure he is creating in you. The video linked in the postscript illustrates this best.