It would help to know more about how your run is constructed (materials etc), or pics.
But, one common reason for wibbly saggy wire fences is if you have not braced the posts (just stuck them in teh ground, each one free-standing). If you do not have either VERY GOOD diagonal bracing of the corner posts, or (better, for a small run) a horizontal board running all the way along the fence near the top of the posts, then you can retighten the fence as often as you want and it will still sag, b/c you are just easing the posts over when you pull on the fencing. (And the weight of the fencing, and the pressure of wind etc on it, will do that even if YOU aren't)
So, connecting the tops of the fenceposts with boards helps a lot.
Also if you use chickenwire there is a real limit to how tight you're going to get it; diagonal stretching during installation helps, but ya know. Even square or rectangular fencing is hard to get really tight if you have a long distance between posts.
Good luck, have fun,
Pat