Catan strategy is as much about judgement calls as it is about heuristics. That being said, I have found that there is one heuristic that can help you through many of the judgement calls you face: be unblockable.
Being unblockable simply means that other players have difficulty hindering your progress. The most powerful way to accomplish this is through board position. Self-sufficiency is a large part it. I cringe when players advise forgoing a resource claiming they can get it through trades. If you’re relying on other players for a key resource, all they have to do to stop you is stop trading you what you need. You’re too easily blocked and it’s going to be very difficult to win if you’re expecting another player to trade it to you.
Self-sufficiency, meaning you have a way to get everything you need on your own, is only the first part. Of course, the only way to be truly unblockable from the robber is to be on two sources of every resource. This would require you to be adjacent to 10 total hexes, which is unlikely. If it occurs, it’s likely it’s so close to the end of the game that having it is fairly inconsequential. The more important thing to focus on is to be on multiple sources of key resources, particularly wheat and ore.
Being on multiple sources of wheat is a key enabler. I wrote in the 10 Catanments post about the importance of wheat in allowing you to spend resources and help avoid sevening out. By blocking your wheat, you’re losing not only production but you’re also likely to lose cards by sevening out on your hand full of unspent resources. So, rule #1, if possible get yourself to multiple sources of wheat early in the game.
In many cases, you will be forced to sacrifice too much in order to produce wheat on two spots, particularly with initial placement. This is where the judgement comes in– in general, it’s not worth it to sacrifice another resource entirely just to get on two sources of wheat. But that being said, it should be the first thing you look for once you do have all 5 resources.
Now, I mentioned earlier that one of the difficulties in having your wheat blocked (or heaven forbid, not producing it at all) is the difficulty you have spending resources. So another way to be unblockable is to have an effective use of ports.
A useful port is nearly a necessity. It’s just too much to expect the dice to give you exactly what you need when you need it. For a balanced strategy 3:1 ports are invaluable. They drastically reduce your risk of sevening out as there is no 7 card hand you cannot build something with, so it’s easier to keep your hand size down. Aside from the 3:1, one of the most powerful strategies that can arise is the ‘dual port’ strategy. When you have access to two 2:1 ports of resources you produce in fairly high quantities (5 pips or higher), you will be nearly impossible to block. Whereas a single port strategy is easy to block by simply blocking that resource, a dual port strategy always leaves a ‘wild’ open. I won the 2013 Capricon semi-final with a highly effective dual port for wheat and sheet (I am red below):
The last way to be unblockable is to simply block other people instead. Knights can be some of the most valuable cards in the game, especially at a table that knows how to use the robber effectively. In my 2012 Masters tournament semi-final, I collected a couple knights early and used them to block a 6 ore hex that two other players were on. This was their only source of ore, so by keeping it covered I prevented them from buying development cards to uncover it. Anytime a 7 was rolled and the robber was placed on me, I moved it back on my turn. As long as I could keep a queue of 1-2 unused knights, I could essentially ensure that I stayed unblocked for most of the game. Here’s me taking my winning turn, you can see my queue of knights and the 6 ore currently blocked. http://www.flickr.com/photos/levimpls/6024394695/in/set-72157627266840553
Despite the many strategies for doing so, the heuristic stays the same. Be unblockable, and you’ll have a much easier time winning even when everyone is out to try to stop you.