While everything Ed said is true, I generally do it this way; when I first fly the plane I get it to a safe altitude and in a straight and level flight path (preferably on a day with little or no wind), then release the controls. If the plane tries to roll slightly to one side, I'll adjust the aileron trim a bit to compensate. Same for any of the surfaces.

Unless you're flying IMAC, 3D, or Patern, chances are once set, you won't need to adjust trim again.

However, if you had to add more than a click or two of anything, you should mechanically adjust the clevises so that the transmitter trims can be reset to neutral. A computer radio can "remember" the trim settings for a number of planes, but the servos will be slightly off their neutral setting, and have a bit less throw in one direction than the other.

Once you've flown for awhile, you tend to compensate for slight trim changes during flight automatically. ie. your finger compensate without you realizing it. (Or mine do.)

Simplified version, bit hope it helps.
Dennis-