So far I have found that with my iPad on my kneeboard that any glare can be minimized by tilting the screen one way or another. Having brightness set at the higher levels helps a lot too. Try it without a screen protector first, you may find it's good enough!
The only screen protector i've had that I really liked was one that I used on my old iPhone, and I no longer have the box for. A lot of screen protectors are kind of a "rubbery" material, that can stretch/get nicked/etc. if you aren't careful. And they're very prone to getting bubbles during installation. The one that worked best for me was a harder plastic, it felt more like a sheet of overhead transparency film. It had a super-smooth layer on one side that seemed to stick to the iPhone glass like static cling. There wasn't any adhesive I could see or feel, just the two really smooth surfaces bonding well enough to keep them in place.
If I could find something like that again for the iPad, I may give it a try.