You should leave it as upright as the conditions allow (resting spot/support, wind, etc.). The closer a rod gets to horizontal, the closer it is to getting broken. I typically nestle it into some outer limbs or busy branches, though wind can be a factor.
As
@Coach Potter mentions, you want to try to leave it near something that ideally stand out to you from a distance, but if nothing else will at least trigger your brain when you're near it again. Whether that's a particular bush, tree, rock, bunch of flowers, notable clump of grass, etc. Just something that let you know you're near. If it's right as you approach the water for the first time (for a given stretch), that can also help locate it--"I came down the bank right about there, so the rod should be near that spot."
I have thought about carrying a strip of flagging tape that could be loosely tied on to the rod and then easily taken off and put back into the vest/pack.
The very worst--and unfortunately easiest--thing to do is to approach the water, get excited about fishing and start focusing on the water, and then just casually deposit the 2nd rod, thinking
oh I'll remember where that is. You wont. You have to make a distinct mental note. Particularly if if might be hours before you're back for it.