When reading Eric "Rizen" Lynch's bolg (famous online MTT pro) I came accross this handy nugget of advice:
"So, how do we know who has a lot to lose from playing a pot with us if we don't have a big stack? A general guideline that I like to use is that any player who has 15-30 big blinds and less than twice my stack size is a prime target to attack. If players have at least 15 big blinds, they are not so short that they're desperate to double up, and if they have fewer than 30, they typically are shallow enough that they're reluctant to take flops with people without a real hand."
Just to put it into context (http://www.rizenpoker.com/articles/2007/08/target-practice.html), he's talking about the mid stages of an MTT, and who you should apply pressure on pre-flop by blind stealing etc. Attacking the mid-stacks is old news but I thought it was a handy rule of thumb :)
That's all for now, can't wait to get his book "Winning Tounaments One Hand at a Time" in 3 days!