I know you can't do everything but here are some ideas.
I want to suggest that good rallies will have two signs at every right turn and
three signs at every left turn. The third sign sign could be a wrong way
sign. Also there should be a "straight ahead" type sign at every intersection or at least the major intersections.
One ride tied streamers on trees and signs. Another used the little construction
flags. Also put out warning signs for road hazards such as potholes or narrow bridges. You can't mark everything but you can try. There should be some kind of signage no less than 5 miles apart.
Do not just put up some signs and forget them. Signs will get stolen, moved or
just blown down. Have the sag drivers or somebody keep an eye on the signs.
This comment was so extensive I created a page just for it. Notes on sinage
I always try to ride in a paceline on every distance ride. Unless I am taking my turn in front, I am generally spending MUCH more time looking
at the bike in front of me than at anything else. I certainly look ahead frequently to make sure I'm not following the group over a cliff, but
there is almost no chance that I will notice an arrow painted on the road unless
it is REALLY BIG.- A sign raised on a stick, on the other hand, would be hard to miss.
I suspect that many folks in the paceline would notice it and announce the turn. People riding by themselves would also be more likely to see
this. - So, my recommendation is for ride organizers to buy some poles and pay to
have plastic signs made (or otherwise waterproof their signs) and post them at all turns. A sign telling riders to go straight thru intersections
where appropriate would also be nice. Lastly, whenever a split in routes occurs
(62K go left, 100m go straight, or whatever), a heads up sign 100 yds or so before the split would help riders get on the correct side of the group
before the intersection.