You take a turn at the front for 30 seconds. Then pull off to the left & ease up so that the rider who was behind you is now pulling. You drift to the back and get behind the last guy until it's your turn at the front again. If the pace is too high for you to take a turn at the front, then stay at the back and let the guy who's drifting back cut in front of you by leaving a gap for him and telling him "go ahead" or something so he doesn't think you're just not paying attention and letting gaps open.

This is assuming you're in an organized paceline. Disorganized pacelines are more common.