Timing your attack

[Drisos]

Sometimes though, if you wait a bit, watch what's going on on the map, and then pick and choose ONE ally with mutual enemies and never attack them, and support them with troops when they attack mutual enemies, I have found that sometimes (and only sometimes) they will actually remain quite loyal to you--until the mutual enemies are destroyed and there's no one left for them to attack but you .... This can be very helpful if one of the other clans is very strong (like Imagawa tends to be early on in the Sengoku and 1530 scenarios); you can leverage the strength, position, and flow of your ally against them.

I second that... I always used to play Uesugi or Takeda, and team up with the other one of the two (Uesugi or Takeda) against the Hojo and the Imagawa.