Strict enforcement of carry on limits is all that is needed. Couple that with significant fees for gate-checking bags which are too large (or too numerous) to carry on is all that is required. The market will take care of the problem.

On WN it's even easier to assure compliance. If you are pulled out of the boarding line to deal with a gate check, you lose your boarding position. It doesn't take long to go from a nice aisle to a lousy middle.