I usually shoot for variety--if you can cover all the numbers, you'll be getting SOMETHING on every non-7 roll. OTOH, if you try to monopolize the 6 and 8 hexes, lady luck tends to sh!t on you and they never roll up.
Additionally, variety of resources is also good--the sooner you don't "need" the help of the other players to get a leg up, the better.
Another thing I try to do is become a tycoon in one element, AND get a settlement on the matching port. It can even be worthwhile to put one of your first settlements on a port.
Worry about your other opponents, but don't go out of your way to cut them off. All too often, the people I play with spend so much time trying to cut off the other players, that they spend full-on 3 settlements' worth of resources just building roads. If another player is encroaching on where you want to build, you may have to race him, but don't race him if you don't need to.
Resource cards are your friend. If you can run away on soldiers, or amass a bunch of point cards, you're that much closer to winning but nobody knows for sure--You're playing to 12 points, you've got 9 on the board, no one really sees you as a threat as much as the other player who has 11 points. While everyone is beating the crap out of that player, you wander in and build another settlement and toss out your 2 point cards and BLAM!
And finally:
Never, but NEVER keep more than 7 commodity cards in your hand at any time if you can help it. Don't wait until your next turn to trade those 4 wool cards in for a brick--tomorrow will never come for you.