I would start by being willing to fork out the money to actually get decent writers and actors. Maybe pair Salvatore or Hicks or another one of the writers whose done some good D&D novels with an experienced screenplay writer. And be willing to let them go a little darker with no goofyness or D&Disms. Basically make it a fantasy adventure movie but not neccesarily a D&D movie to the casual observer. So like someone said up above, no "i cast this" or other things are obvious game rules stuck into the movie.
More then anything it seemed like the last ones suffered from bad actors and too much cheesy filling though. Fixing those would probably make a huge difference, maybe enough of a difference to actually get a theatrical release instead of a made for TV one like last time.
D&D 3.0 and 3.5 each had a movie to go along with their releases. I'd wonder why there is no similar movie for 4E's launch but its an easy guess that they previous two movies didn't make enough money. But if you were in charge of coming up with the basic concept of the 4E movie, what would it be?
I'm working on a spec screenplay that features gamers who play a D&D-like game (without being D&D of course - I don't exactly have the license from WOTC and don't expect to get one). I don't want to reveal much of the plot, but the gamers end up using their game knowledge to save the real world.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.