Out of habit I carry two bodies "in regular use" and three when on a trip. Usually wide,normal (35/40) and a 50/75.
The bodies are all the same mount, usually M-mount and one body serves as a back up in case one of the two working bodies acts up. Depending on where and for how long I am going (one week+) it would be a Bessa R4M for wides (21 and/or 28), a M2 for 35/40 and a MP for 50/75). It all depends too on where I am going, Europe has narrow streets and alley's so the 21 is occasionally supplemented by a 15f4.5 ( it is small enough that it can rattle around in the camera bag without problem). At least one of the lenses would be a higher speed f1.4 or f1.2 - usually the 35 as that is my primary lens.
Everything is tried out before going, bodies have a roll or two going trough them, lenses are checked out and any batteries are replaced with fresh ones before leaving - and even the spares are checked as batteries are prone to discharge or die in its packaging.
The trick is to establish a "standard" kit and keep using that until it becomes second nature. Nothing then worse looking like a tourist. peering at a small screen on the digital or trying to change lenses in the middle of a busy piazza in Italy.
AND never park the rental car, open the trunk and sort through stuff. Always stop a couple of km away, sort out the kit and remove anything looking "touristy" from the interior of the car (maps, jackets etc) and leave the view of the interior clean.
If you are a film shooter. estimate your consumption of film and double it! Standardize on the film too and get to know it before leaving. Experimenting belongs at home, not on a trip you have forked out big bucks for.