One of the rules out here is to keep a sharp lookout at all times.

Fine, so all you need to do is keep a sharp lookout.

He kept a sharp lookout, making note of the landmarks they passed.

Have them back before dark, and keep a sharp lookout.

I dare say there is, Doctor, if you keep a sharp lookout.

Then he warned them again to keep a sharp lookout for airplanes, particularly to the west.

You get to the house and keep a sharp lookout.

The rest of you stay here, but keep a sharp lookout.

Keep a sharp lookout for the next hour to the east.

Everyone needs to keep a sharp lookout in case they come back.