i saw a pro poker player give this advice on "poker night live" on british TV about 2 months ago and have done it ever since
when in a tournament, be sure to not only observe your opponents actions, but every 20 minutes or so ask yourself questions abuot all the players on your table...start with the guy to your left, and work your way around..this is usually best done while youre not involved in the hand in progress(duh, lol

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you should be asking questions such as:
1) how many hands is he/she playing?
2)is he/she loose/loose-aggressive/tight-passive/tight-aggressive?
3)what kind of hands is he/she showing down?
4)what position are they in relation to you? (left/right) (this is obvious to most but it is overlooked by some to be unimportant)