Expand early

In Settlers, the earlier you build a settlement, the more resources it will provide overall. As such you should first focus on building more settlements before attempt achievements like going for the longest road or purchasing development cards.

In addition to this, you should expand into areas of contention where you feel you have a edge at establishing a setltement befoe they do.

Finally, you should first focus quantity of settlements over quality, while upgrading your settlements into cities will increase resource production, it is generally better to build settlements first to diversify your resources, this is two fold as it also blocks your opponents from having a resource generating avenue.