I have one tip for you..
Credibility - [definition] - believability of a statement, action, or source.
As long as it's believable, the audience will take it in.. the worst possible sci-fi/fantasy movie completely defies natural laws in
ridiculous ways. I think there's a fine line between sci-fi/fantasy and the "cheesy" genre.
P.S. I saw X-Men III and was pretty pleased with it.
