In the early and middle parts of the game when there are still building projects to come I switch builders to another skill once there are no projects actually in progress, and switch them back when a new one is started. One exception might be when a villager is on the verge of becoming a master builder, but that's rare. In any case, it is difficult to see by eye how far away a villager is from becoming a master so there could still be a long wait.

In the late game, when all buiding has been completed, builders will still advance their skills by fixing huts, as mentioned. They normally do this automatically, but they tend to wander off a lot to cool their feet, enjoy the pond, etc. If I spot them doing this I'll drag them back to work. By the way, it is important that their action line reads "fixing" the hut. If they are simply inspecting the hut they will gain no skill. If I see a villager doing this I'll drag them away and drop them back again until the action line shows "fixing".