The stupiest advice that all books, all pros, all your friends and everyone tells you is: PUT THEM ON A HAND. BS. I'll put them on a hand for you: complete junk, bluff or straight flush nuts. Hows that for putting someone on a hand. I''m right 100 per cent of the time.
Right, you don't put them on an exact hand, you estimate a range. Then as the board plays out you can narrow their range accrding to what is on the board, betting patterns, player tendencies and histroy until you can narrow their range to a few possibilities. If you think that they all come down to "complete junk, bluff, straight flush nuts" then you are not paying enough attention to your opponents and therefore will never get any better at playing agianst them.
The second stupiest advice is: It's not the cards but how you play them. BS. It's the cards. To be quite honest, it comes down to those little guys running around in your brain: Mr. Fold, Mr. Call, Mr. Bet, Mr. Raise, Mr.Reraise, and most of all Mr. Allin.