Walls and Ceilings - Locating where to cut holes for can lights

I was wondering if anyone could share some tricks/procedures to locate exactly where to cut the holes in the ceiling sheetrock so the hole match the location of already installed light can's?

There are two ways. First measure the center point of your can. This is done by measuring from a parrallel point and a perpendicular point. That is measure off the wall in front of you and from the wall (or end of other sheet) to your left or right. Transfer your measurments to the sheet of drywall. Unfortunately you will most likely miss because of unsquareness. The second method is using a trim tool like a roto zip. First measure to the center of your can from one point (wall in front of you) next hang your sheet. Screw off the perimiter only. Next find you center mark and zip away.

Thanks for the two great proposals.

I actually also developed a third method which I want to share with everybody.

Using a plumb attached to a string - and hanging it from the center of the light can - I first marked an x on the floor. I then fitted the sheetrock and again used the plumb string to transfer the x back to the sheetrock.

After that it was a simple task of using a hole saw. It worked very well.