I have a new strategy that I tried this weekend, and it worked. I won the game on my first try using it.
You know how when you don't buy development cards, you end up with all these sheep in your hand that you don't know what to do with? You only need one sheep for each settlement. Well, I found a perfect way to utilize those unwanted pesky sheep.
My strategy is to capitalize on sheep. Put both of your starting settlements covering all 5 resources, 2 of them being sheep (on different numbers - this is important). Make sure your second one is a good number (6 or 8) and your other one can be a back up.
The settlement that is closest to a 2:1 sheep port, build roads ASAP to that port and place a settlement.
Quickly turn your settlements on your sheep farms into cities, which will not be difficult because you can trade 2:1 for the resources you need to do it. Then just continue to trade sheep for everything that you need to build roads and cities, mostly; to get 10 points.
BTW - With this strategy you need to not buy Development cards because this is just a waste of resources.
Also If you can upgrade all settlements to cities (5 resources) since this uses less resources than building 2 roads and one settlement (8 resources). You can have 10 points by only building 4 cities, 2 settlements, and 6 roads joining them in 2 separate star patterns. (If your sheep port is in the right place)
My friends and I have played a lot and I've only seen the sheep strategy work once, and it was due to fortuitous placement of the sheep harbor (we would have all been crazy not to go for it on that particular map).