Lingerie is always the best way to go. The allure of what's hidden is always greater than full nakedness. It also helps to accent the best parts of your body.
One thing you want to make sure is that lighting is good. Shoot a few test shots just to see how the picture is turning out. Get yourself a neon/diode white lamp or do it with daylight near the window - those lights are best and help to make the colors right.
Also if you use the lamp you may want to go for indirect lighting. Like shine it at the wall or something big and white, so that you are illuminated by the reflected light. Because if you point a lamp at yourself there will be really odd shadows behind you.
Make sure your room is very clean behind you with as little detail as possible.
Now for what you should do. I think it depends on the length that you are aiming at, but I would say the best thind would be to sit in a chair (or on your bed with pillows behind you) with your legs spread. If you go the chair route make sure it's stable and you can use other chairs to place your legs upon.
THis is the easiest position that lets you both face the camera and demonstrate all your goods.
make sure your camera is above you and looking slightly down on you. This way psychologically you look more submissive, less threatening, and overall impression of the video will be nicer. That is, if he's not into femdom - in that case do reverse.
Smile, but naturally. Look directly into the camera at least occasionally.
If you can, then do sexy undressing/striptease. If you can't make it work - don't bother. =) Better skip it than produce an awkward video.
Act natural. Don't go the dirty talk route if you don't feel like it. For me, personally, the ideal thing would be a few worda at the beginning of the video, where you say "Happy birthday" and explain your idea, maybe throw in a few phrases that you feel like saying like "I feel so horny doing this..."
Then as you get deeper into the action I suggest you stay silent and just make sexy sounds. Again, men are different, but I personally find it distracting when women talk too much. But the most important part - stay yourself. Don't force it. If you feel like talking - talk, if you don't - just let your breathing talk for you.
As for if you should expose pussy or not - for me ideal would be starting the video with it hidden but taking it out later, towards the end or as you start to seriously masturbate.
As for the "script" of your video I'd go for this:
0) dress in lingerie
1) You are sitting in your seat, lingerie on, panties on.
2) You talk about this being his birthday present and how you think of him a lot and so on. I suggest you practice it a bit so it sounds natural. If your breasts are not exposed - take them out at this point (if your lingerie allows it, if not - no problem Leave them hidden and play with your pussy while it's still hidden by your panties.
- this part shouldn't be too long, 20-30 seconds will do
3) You finish your speech with "Happy birthday, enjoy your treat" and at this point either shift your panties to the side or take them off. Or, if you chose so, just leave them on and masturbate like that.
Alternative script - more naked.
0) you start with your T-shirt on and some mini-skirt. Ideally - thigh-high stockings too.
1) You talk your speech completely casually. You explain that this video is a present for him.
2) As you finish your speech and say "Happy Birthday" - you roll your T-shirt up to expose your naked breasts, you spread your legs and pull your mini-skirt up, and then do your thing.
And I do think that you should watch it together, preferrably when you have time alone. This way:
A) you make sure he watches it
B) it's more intimate
C) There's a chance he gets to make love to you after watching it after all.
One advice: don't look at him while he watches. Be at his side and look on the screen even if he looks at you. If you look at him this may stress him out because you obviously expect reaction and at this point he may be not sure which he should give.