The choice of connecting cable is important. If you can locate it, double shielded mil-spec Teflon coax is the best. The new Times coax with a metalized mylar shield works very well. The coax does not have to be large diameter if you are running under a couple of hundred watts. The lengths are so short that losses are not a consideration.
Connectors Pay careful attention to the duplexers connectors.
Most of the mysterious noise problems I have heard in repeaters have been traced to the coax connectors. I have had minor problems with noise that was easily cured by cleaning and tightening the UHF connectors. If I had a choice, I would use N or BNC connectors on any new duplexers. N series connectors are the best and well worth the extra money. There is a difference in connector quality.