Don't wait for a gap. Shoot through people. Let them partially obstruct your subject. Heads and shoulders in the foreground create depth and atmosphere. A subject half-glimpsed through a crowd is more interesting than the same subject in empty space.

Watch for: the subject becoming completely lost. You want partial obstruction, not total. A visible eye, a gesture, a flash of colour is enough.

The point: crowds are not obstacles. They're material.