I think first position is the #1 factor in stealing Blinds.
Then I would say the cards in hand.
Thirdly, The Chipstacks of the players in the hand.
The Style of the Player.
Also there is the big factor of when you do it.
The later in the the game obviously the more the steal will be worth.
When the blinds are better percentages of the chip stacks in play, after the increase that effects a player on your table.
Learning how to make steals with full tables are the key to long tourney play. Try to get into a position with 1 caller prior to your raise.
Try to make sure it is one of the weaker players at the table, who either is not playing anything but primo, or the guy who has been low all tournaments and has not seemed to get any luck in this tournament.
I don't like to steal from "lucky players" usually they are loose, or seem to have the tournament on their side.
Stealing blinds online is much different than live. Chip stacks seem to be favored a little more online, the
hands seem unreal.
Playing less gives you more creditable positions to raise. Due to players viewing you as tight. So sometimes playing a lot less in middle blind rounds, can take you further in the tournament, and give you that edge in stealing blinds in later rounds.