Hoarding development cards and developing self-sufficiency, as mentioned by other answers, are two good common strategies I've seen employed often in Settlers. They do work, but a highly cooperative defense can stop even a fairly self-sufficient leader with 1-2 random development cards. Sometimes these will be the only options available to the leader.

However, I've found that "sneak attacks" work even better, when they are available as an option. What all sneak attacks have in common is that you purposely do NOT take the lead, but rather stay just a point or two behind the leader or perhaps convince other people that even though you're tied for the lead that the other person is more likely to win. Then, when the right circumstances hit, you leap to victory on a turn where you start with only 6 or 7 victory points.

There are a number of ways to do this. Here are some more common ones:

• Hold off using a Monopoly card even if you get it very early in the game. Wait for exactly that right moment when there are many of one resource that you can monopolize and efficiently convert into several settlements/cities/dev cards.
• Hold off using a Road Building card until it results in you getting longest road and a new settlement all on the same turn.
• Hold off using a Year of Plenty card until you happen to have a card combo in your hand where employing it results in a 3 victory point turn.
• Have a couple roads that go off to horribly unproductive locations - but are nonetheless potential settlement sites. Many people automatically assume you'll never build there - but you will on the turn it vaults you to victory.
• Sometimes people have a hard time seeing how your disjoint road network can turn into a longest road by closing certain loops. This one is actually the most common type of successful sneak attack for me - people often have no clue that I can take the longest road with just 2-3 more road links closing 1 or 2 loops.

If you have to spend many turns defending against endless robber attacks and overly cooperative trading, you may or may not win no matter how good you are. On the other hand, if time a sneak attack right, you just win.

Hoarding development cards and developing self-sufficiency, as mentioned by other answers, are two good common strategies I've seen employed often in Settlers. They do work, but a highly cooperative defense can stop even a fairly self-sufficient leader with 1-2 random development cards. Sometimes these will be the only options available to the leader.

However, I've found that "sneak attacks" work even better, when they are available as an option. What all sneak attacks have in common is that you purposely do NOT take the lead, but rather stay just a point or two behind the leader or perhaps convince other people that even though you're tied for the lead that the other person is more likely to win. Then, when the right circumstances hit, you leap to victory on a turn where you start with only 6 or 7 victory points.

There are a number of ways to do this. Here are some more common ones:

• Hold off using a Monopoly card even if you get it very early in the game. Wait for exactly that right moment when there are many of one resource that you can monopolize and efficiently convert into several settlements/cities/dev cards.
• Hold off using a Road Building card until it results in you getting longest road and a new settlement all on the same turn.
• Hold off using a Year of Plenty card until you happen to have a card combo in your hand where employing it results in a 3 victory point turn.
• Have a couple roads that go off to horribly unproductive locations - but are nonetheless potential settlement sites. Many people automatically assume you'll never build there - but you will on the turn it vaults you to victory.
• Sometimes people have a hard time seeing how your disjoint road network can turn into a longest road by closing certain loops. This one is actually the most common type of successful sneak attack for me - people often have no clue that I can take the longest road with just 2-3 more road links closing 1 or 2 loops.

If you have to spend many turns defending against endless robber attacks and overly cooperative trading, you may or may not win no matter how good you are. On the other hand, if you time a sneak attack right, you just win.