Rear hatch rust prevention

I bought my 95 lwb in San Antonio when it was 5 years old with no visible signs of rust on the rear hatch. I brought it to Kansas and a great deal of rust developed after the first winter here. I just bought a used, virtually perfect replacement out of California this week and before I put it on I want to rust proof it and avoid this from happening again. Has anyone had any success in keeping their rear hatch rust free in the midwest where it will inevitably deteriorate. I am not opposed to taking it apart and rust proofing and repainting this for longer life. Also are there any other tricks to avoiding rust. Someone once mentioned drilling relief holes in the bottom to allow water to escape. Is this a good idea? and is there anything else I can do.
Thanks a million for your insight.
Scott

use windshield sealant, it is applied with a caulking gun to the frame. ihave done it to all three of my trucks and definitively extended the life of the tail gate.
it is made by 3m you can find it anywhere, the caulking gun at your hardware store, home depot or lowes. i tapped with masking tape around the frame on the very edge and on the glass next to the metal then applied a bead all around the glass were it meets the frame on the out side and then just for safety I took my gloved index finger and ran it around the frame forcing sealant into the edge. it looks messee on the tape but when you peel it off (inmmediately after you are done) the edge looks very profesional (btw, I used latex gloves).
good luck.