I occasionally have to trim my gears as well.

Just a minor 'up' or 'down' on the rear derailleur gears (left hand side shifters (versus right which is your front derrailleur).

I'm not 100% certain why trimming needs to occur but I do know that it is common. Whether to trim up or down is dependent (at least for me) on whether I'm coming into an easier gear or going into a harder gear.

I'm not sure what kind of shifters you have but I just give either the 'higher gear' or the 'lower gear' shifter a slight nudge (usually just an audible click of the gear) and it settles it all out and everything is smooth. Don't worry if you trim the wrong gear. If it doesn't work, try the other gear.

If you trim too heaviily, the worst thing that can happen is that you'll actually change gears completely as opposed to smoothing things out.

I hope I didn't confuse the crap out of you.