What's a good rope signal system?
Climbers often need a backup method to communicate to their belayer when they climb out of earshot, usually on a multi-pitch. The shape of the terrain or high winds can make communication by shouting impossible.
Since the only bit of information that a leader generally needs to communicate to their belayer is "I'm safe", it makes sense to use a simple rope system that only expresses this one declaration.
A good protocol that many climbers use is the "seven means safe" method: Seven strong and even tugs on the rope by the leader is equivalent to issuing the "off belay" command, or indicating to the belayer that the leader is safe.
A few tips for using this method in practice:
The leader should make the first tug a strong one, so as to take up any slack in the rope system and -- in the case that a Gri-Gri is used to belay -- lock up the belay device of the belayer. At the very least, the tugs should be enough to capture the attention of the belayer so that further tugs don't go unnoticed.
The leader shouldn't simply pull against the weight of the rope, rather they should pull directly against the belayer. Extra slack in the system will simply cause a standing wave in the rope that might not be noticed as tugs by the belayer.
Whether the leader tugs seven times, eight times, nine times, or twenty times doesn't really matter. The point is that if the belayer feels a lot of strong tugs on the rope separated by a regular interval, then this is an unmistakable signal that the leader wants the belay off.
If the belayer is unsure of the intentions of the leader, she should leave the belay on. It goes without saying that under no circumstances should the belayer pull on the rope themselves.