First you want to profile each player, at least roughly. Are they tight or loose, agressive or passive?

An important part of reading is figuring out what their poker mentality is. Do they live and die by TPTK and over pairs? or are they smart and know that AA isnt always the nuts on the river?

As others have said before, one of the most important elements of reading is noticing betting patterns. Do they like to slow play their monsters? (I concider this one a pretty important thing to notice..it could save you alot of chips later) Always be checking hand histories and watch all showdowns, especially big ones. Do they show down marginal hands for big bets? Are they overly agressive. Especially take note of showdowns on which they raised preflop.

Also, some disagree wth this, but I think a player can get good reads off of the timer...i.e. how long a player takes to make a decision. Think/call means weakness alot of the time, while fast decisions usually means a made hand (or maybe a quick-thinking player, which means hes good). There is alot more to be written on the subject of timers, but I dont really feel like going inidepth...

Also instant folds usually mean they are using the preaction buttons, which is an indication of multitabling, which can indicate a better player (one who feels comfortable mutlitabling, because you wont find many fish multitabling)