Metagaming: using the knowledge of the player to determine what decisions the character makes.
Avoiding metagaming, step by step process:
1) Determine what the player knows... and in doing so, metagame.
2) Force the player that knows what is a good idea to jump through hoops like die rolls for knowledge checks or do something they know isn't a good idea - even though a player that had no clue and was guessing is allowed to play their character as making a guess, thus proving it was the player's knowledge - not the character's - that was used to decide the course of action.
You can call it "bupkis" but I have proven it as being true, including that if I recall correctly you and I actually had a conversation on another forum about how I'm not allowed to have my character guess about something in one of your games because I know as a player that the character's guess is correct.