My time-honoured motto for star trails is never expect perfection; it could well come unexpectedly: you are out there to have a bit of fun, not to make an exact science of photographing the heavens, but just for entertainment. With good planning and an awareness of your position relative to the stars, good results will be a given. But never ever be discouraged or put off by blank frames or even falling asleep (Hoffy has it right by having a hip flask at the ready!!). And do experiment with film, like tungsten film that gives a blue, daylight-like hue to the scene (my next trick).