Among the other good answers, this stuff is excellent.
http://guitararmor.com/
It is tough, virtually invisibly clear and (with proper prep) will stay attached forever. It is very similar if not identical to the clear protective material on my and my kids' bikes (e.g. chainstay protector). It is also similar if not identical to the abrasion patches used on my Subaru to protect from contact points of my hood/bug deflector. It comes in sheets, but can be easily cut to any shape and size you want.
I happened to have a partial package because I have actually used it to make a clear pickguard for one of my guitars, so I tried it.
For about $10 you can have a large supply of this stuff for everything from chain protection on the chainstays, to cable protectors, to protection on areas that might rub during transport or on the repair stand, etc. etc.
Another thing I've used many times over the years which works pretty good is clear packing tape. It isn't as tough as the above stuff, but actually works pretty good and stays attached remarkably well.