Trees. Trees are (quite literally) books or paper.
line 1: jungle gym trees
2: paper
3: long trees, short trees
4: fat trees, skinny trees
5: I hope I'm not barking up the wrong tree with this answer
7: When they try to hide behind one
8: Paper currency...
To clarify, strategy is two parts: days 1-100 use even/odd method and designate Counter (second repeater);
days 101 till end is "regular" (post#2) strategy.
They won't know who Counter is, but they'll know who it ain't (them).
When the first second repeater leaves room, he resets to original...
Begin with Phlegmy's #71 post of even/odd days for (say) 100 day intervals. The first repeat prisoner will reverse the order. The SECOND repeat prisoner becomes counter and begins "regular" (post #2) strategy, starting at the next 100 day (100 is probably not optimal) interval.
(There MUST be...