[Base game]

The biggest part of the game revolves around challenging your opponent's cities (or roads) and preventing him to steal yours.

Since the base game has a pretty restricted set of tiles, blocking enemy meeples is quite easy.

A city with only one meeple is vulnerable to an opponent's takeover, but one with several meeples is vulnerable to blocking tiles. If your city becomes impossible to complete, your meeples will be imprisoned here, which is a big handicap.

For those reasons, avoid building giant cities, and prefer several small ones instead, so that they are not too interesting to attack for your opponent.

Also, try to avoid using too much meeples at once (e.g. building 3 different cities if you could focus on one and finish it first).

In fact, you should try to always keep at least 1 meeple in your hand until the last few tiles.

[Traders & Builders]

Make sure to use the builder as early and often as you can. Only avoid using it if your opponent will likely be able to trap it.

Focus on building cities as much as you can. Roads are already quite weak in the base game, but with extensions, and the "resources" on cities, they are even weaker.

For the same reason, it's not always worth putting a meeple on a cloister : will you really be able to get it back ? Is the risk worth only 7 or 8 points ?

Don't waste the big meeple on fields (except in the very end, of course). He's particularly precious, both for attacking and defending cities.
Also, try to avoid having him trapped, of course. It's much more inconvenient than losing a regular one, really.