For the most part, I ignore metagaming. I figure it's part of the character's training, or something he remembers from a story he heard, or whatever. If you lived in a world of dragons, and it was your job to go out and fight monsters, don't you think you would know which colors are immune to what?
Real life example: if you get an email from a Nigerian prince, do you consider it meta-gaming life if you don't take him up on his offer?
Or is it something your real-life character just happens to know?
I do draw the line at players opening the monster manual in the middle of battle though.
I think the biggest metagaming I see is when a new character joins the party. Such a person is always treated *way* different than any NPC the party meets.