If you want you alliances to last longer, and it is with a neighbor, try not to build up troops excessivly on your shared borders. This is not to say that you should leave yourself undefended, but try and have a similar amout of troops stationed.

If you start disarming along a border, and your ally does as well, then its more likely to be a lasting alliance, but if your ally maintains a large garrison on your border, its most likely they will be attempting to invade your lands relativly soon, or sometimes it is because they are at war on another provencial border. Either way, a well defended provence is a good deterent for invasion in this case.

It also helps if you are at war with the same faction. For instance, France and Italy vs. Germany.

Be aware that if you end up at war with another faction your ally is alligned with, the factions tend to ally with the weaker faction, or sometimes the ally they've been with the longest.