Or stick a cylinder through the neck seal over night.
It should be reasonably tight but not to the point where you go a funny colour.
A couple of words of warning, some lubricants will rot the neck seal, KY jelly will not perish the latex seal.
If trimming it then you need to trim off a fraction at a time. The small amount you trim will make quite a difference all the way around.
If you bought it from a dive shop, go back and see if they will trim it for you if neccessary.
Finally if you do trim it yourself, then as said before make sure it is nice and even, no nicks that can tear. Find the sharpest implement possible for trimming and then look around for something even sharper
My LDS use a pair of extremely sharp sewing scissors.
HTH
Daz