There are a couple of ways. You could get one of these new-fangled wire winders with a ratchet action. You put it on the fence wire, wind it up and pull the wire tight.

You could use a fence stretcher, cut and splice the slack out of the wires.

The wire should be held tight by the clips onto the T posts.

Yearly adjustment shouldn't be needed. If it is, I'd be wondering which fencepost is coming out of the ground.