I think the main thing you need to do is correctly identify your strengths and weaknesses, and don't try to force a strategy you're not set up for.
If you don't have abundant brick and wood, don't even bother going for Longest Road.
Even the worst placement will have some resources they are rich in.
Try to get a port that capitalizes on your resources, and make the necessary trades with other players to get the resources you're poor in.
If you're rich in... Brick or Wood -- try for longest road and build plenty of settlements Grain or Ore -- upgrade your starting settlements to cities and buy development cards Sheep -- go for Largest Army via development cards, or depending on your next best resource, you'll probably follow one of the other two basic strategies Of course even with all of this, sometimes the dice just won't drop your way :)
boardgames.stackexchange.com/users/32/lilserf
If your initial placement is bad, hopefully you can still build a little.
The first couple road placements are critical.
By then you should be able to figure out which commodities are in short supply this game .
Either because they haven't been rolled yet, or they're sitting on a 3 and an 11.
Use your first two roads to build for that scare resource, claim it, and then see who will overtrade you for it!
boardgames.stackexchange.com/users/40/Pat Ludwig
If you find you're often badly placed, initially, it may be that you're either making errors there, or misjudging where other players are going to place their initial settlements.
It's easy to pick your optimal placement when in a four player game you place fourth-fifth, and you can usually figure where the fourth player to play will go when you place third-sixth.
If you go first you usually grab one of the two best producing junctions, and should aim for brick and wood since if you have those two with your first settlement, if your options are limited when it gets back to you
at least you're able to move, and other players are often hungry for brick and wood early in the game, meaning you're the trading king.
Going second-seventh is often the toughest slot, since the first player to go typically grabs the best brick and wood combination (assuming there is one), and there often isn't another good brick and wood combo on the table.
Then, of course, by the time it gets back to you pickings tend to be slim.
Still and all, given the good advice people have offered, above, you really shouldn't run into setup situations too often at the beginning of the game that seriously handicap you.
One of the three main lines of play should be open to you.
It's unusual to be stuck with setups where all you get are ore, sheep, and brick; or wheat, sheep, and wood; those triumvirates that leave you unable to move, unable to build, and unable to pick up development cards.
Avoid those scenarios if you possibly can, since you're absolutely dependent on the tender mercies of other traders to bail you out and let you progress.
Even so, if that's the best situation available to you, you may need to try to build to the nearest 2:1 port of a resource you're rich in, just to get the flexibility that'll get you back in the game.
boardgames.stackexchange.com/users/967/shawnschirmer
Diplomacy.
During the mid game, constantly point out how badly you are doing, encouraging others to trade with you in preference to other players - even offering you better deals.
This can allow you to develop a couple of settlements and get back in the game while the others are playing stop-the-leader.
boardgames.stackexchange.com/users/2040/xorsyst
Initial placement is critical, but once you've seen the layout, you can devise a strategy that will get you something.
One of the big goals you should be going for is number diversity, so that you're always getting something.
boardgames.stackexchange.com/users/96/Lance Roberts
When there is a valuable resource(the average amount of pips for each resource is 11.6)
that has less than 10 pips, and there is no red hex corresponding to that resource, and there is only a 4 pip for that resource(ie there is only a 9,3, and 12 hex for wheat, then try to claim the 9 and block other people from it) and don't trade with people for that resource unless it's 3:1(and usually if you want to trade 3:1
they have a harbor, else usually at least 4:1).
So if you don't have that resource as initial placement, you should try to get a port and trade
what you have(for example you have 2 cities and they correspond to a 8 and a 9 for sheep, get a sheep port) with the bank.
boardgames.stackexchange.com/users/5665/Zeb
In Settlers of Catan, how do you overcome bad initial settlement placement?  In a lot of games of Catan I've played, initial settlement placement seems to really make or break the game for a player.  Are there any good strategies for overcoming a weak starting placement?
In Settlers of Catan, how do you overcome bad initial settlement placement?
boardgames.stackexchange.com
<catan>
boardgames.stackexchange.com/questions/48
boardgames.stackexchange.com/users/10/ICodeForCoffee
The first road placements are critical. Try to get a port that capitalizes on your resources.  The first couple of road placements are critical.  If you don't have abundant brick and wood, it is not worth bothering going for Longest Road. Even the worst placement will have some resources they are rich in and you can still build a little. Try to get a port that capitalizes on your resources, and trade with other players to get the resources you are poor in.  You may need to try to build to the nearest 2:1 port of a resource you're rich in, just to get back in the game. You should try to get a port and trade. One of your big goals should be going for diversity, so that you're always getting something.
The first couple road placements are critical.  If you don't have abundant brick and wood, it is not worth bothering going for Longest Road.   Even the worst placement will have some resources they are rich in amd you can still build a little Try to get a port that capitalizes on your resources, and tradeswith other players to get the resources you are poor in.   You may need to try to build to the nearest 2:1 port of a resource you're rich in, just to get back in the game. You should try to get a port and trade
 One of your big goals should be going for diversity, so that you're always getting something.