I learned this wonderful registration technique for printing multiple plates at Highpoint and I will be forever grateful. It increased the rate of successfully registered prints considerably. I'm not sure which clever mind invented this registration jig, but I believe my sources got it from Crown Point Press, so I'll refer you to their blog on how to make a registration jig. (I wasn't successful cutting my jig on the jump cutter, so I got someone to cut it for me with a diamond blade. It was quite jagged afterwards, but easy to smooth with a file). The jig consists of two L-shaped copper pieces. The narrower one, or registration bar, is taped onto the press bed with double-sided tape.
The second one, which should be at least as wide as your paper border, is a removable jig.
Every time I put my plate on the press bed, I place it into the jig, which ensures all plates are always in exactly the same spot. I keep the paper trapped under the roller when I change the plates, i.e. the paper never moves and the plates are always in place. This system is pretty much fool proof!