Main Sail Trim by the Tell Tales
There's a great book called Mainsail Trimming by Felix Marks that goes into this in great detail. A lot of it doesn't apply to dinghies (travelers, for example). But one thing does - trimming the mainsail using the mainsail telltales.
The telltalls are on the leech (back edge) of the sail, and when sailing upwind from a beam reach and forward (when the sail is a wing), the tell tales should be streaming, just like both of the jib telltales. But what if they aren't? For the jib telltales, we have the guidelines "move the sail toward the telltale that isn't streaming" or "move the tiller toward the telltale that isn't streaming", and students seem to get that. But I don't think we talk enough about trimming the mainsail using the telltales (I could be wrong here, just my observation).
So (on a beam reach and above), if the mainsail telltales aren't streaming, they're curling to one side--windward or leeward. That's just like one of the jib telltales not streaming while the other one is. Move the sail that direction (as in, ease out if streaming to leeward and trim in if streaming to windward), or move the tiller that direction. Essentially the same thing as you would do to trim the jib.
I've seen a tendency in students to oversheet the main, so I think it's useful to sensitize them to this, and the mainsail telltales are one way to do this.
You can also use the mainsail telltales in a gybe. I went on the water once with Stefano Maffuli and a friend of his who had been head instructor at St. Francis Yacht Club (grazie, Stefano). He said that the mainsail telltales were the best indicator of when to pull the main across in a gybe. They show exactly when the main is back-winded--a much better signal than the jib coming across.