In Settlers of Catan, how do you overcome bad initial settlement placement?
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 :)