I got real good at paying attention with one eye and the other eye on the sails.
The one big thing that facilitates this is easy, centralized sail control adjustment. Have as many of the controls brought to one place; the place the trimmer sits.
Ideally the mainsail trimmer should be able to adjust mainsheet, traveler, and backstay from where they sit. And be able to ask the headset trimmer to adjust outhaul, cummingham and halyard.
The headsail trimmer should be able to adjust sheet and car positions without moving.
50% of good trim is being able to do it without running all over the boat! So I'm not a fan at all of:
Jib/main halyards at the mast
Outhauls on the boom end
Travelers over the compaionway
Mainsheeets on coachroom winches
Cunninghams at the mast
Backstay controls on the transom.
Jibcars on pin-tracks.