You have to stretch it tight. I remember someone recently posting about stapling chicken wire to a 2X4 and tyeing it to the back of a truck or pulling it with a come along or something like that. Wire and tension are natural allies.

You're farm & ranch store has a tool called a "fence stretcher". Much easier to use than attaching your vehicle to it. Try it, you'll like it.

I just googled "fence stretcher" and found all kinds. Would you mind searching & providing a link so we can have a better idea of which one you are talking about?

Google "Stay-tuf stretcher bar puller" or go to Uvalco.com. Actually if you go to your local f&r store the clerks should be able to recommend the right one for you and your experience level and how much fence you are installing. The more fence you put up the better quality tool you should consider. Although, the proper tool always makes the job easier. Stay-tuf is a brand name and Uvalco Supply is a F&R store in my location. They have an 800 number and any of the clerks will be happy to help with your questions. (I no longer work there. I retired in January) Or you can google farm & ranch to find one in your area that handles fencing materials.

Also, you didn't mention how you were attaching the wire to the t-posts. You should have gotten t-post clips when you bought the t-posts. These also makes the job easier. I don't know the size of your job but my goat pens were small enough that I did not need pull posts between corner posts. If your run is long then you need your corner posts at each corner and pull post in the middle. My goat pens enclose about a 3 acre square to give you something of a reference.

I always use wooden posts with braces as my end posts as they'll hold tension where a steel post will bend.
For a short stretch bolt two 2x4's together sandwiching the end of the wire, wrap that around the end post, place a shovel handle behind the 2x4's and use it as a lever to pry them away from the post. You can hold that tight with your body while you staple it in place. pull the top and staple, then the middle, then the bottom.
Leave the wire hanging from just you end posts over night as the sag will remove alot of the stretch then you can come back in the morning and tighten everything up.

If your wire is only wavey at the top, then your corners are probably pulling in. You may want to buy some corner braces/brackets made for t-posts. You can usually find them in a kit packaged together. You'll want to use a level to insure they're straight and brace in both directions.

Pull from corner to corner and clip wire to all your braced corner posts first, then to post in between. If your posts are in straight and wire is stretched evenly your vertical straids in your wire should run in line with your posts. The above idea using 2x4s works well if your corners are straight and braced.

Just remember to get the vertical strands lined-up with posts before you clip them and do all the corners first. This way you can check all corners for alignment before clipping all your other posts.

If your putting in a gate be sure to brace the gate posts also as they will be under tension also.

GOOD LUCK,
MJ

Thanks for all the help!

Oh Gosh I am really feeling like an amateur now. I just looked up t-post clips an found that I have my saggy fence set up all wrong. NO BODY EVER TOLD ME ABOUT ANY CLIPS...
Okay it is back to the drawing board. well anyway it is just a 8x10 fenced in area for chickens to come out when I am sitting outside so no harm done.

don't even ask me how I have mines set up...

Sloping or uneven ground has been our nemsis. We then had to fence in sections.

As for t-post clips, we have NEVER found them to be useful. We have a fence stretcher, we get the wiring nice and tight, but the clip always seems to invite slack. So instead, we use heavy duty, (UV protected) zip ties. Fast, strong, and they hold. Don't like the way the wire looks? - snip! and its free. We can later go back and reinforce with heavy gauge wire...but frankly, our fence is going on 4 years old and those zip ties are still holding just fine.

I used zip ties all over my place. LOL. It does last a good long time but where do you get them for free? I had to buy mines in Lowes

What if I just wrap the welded wire around a 2×4 and stretch it? I don't have a pulley. I'm nowhere near a supply store.