I recently bought a new 3 year old mare from a good friend of mine, for a very good deal. The first few times when I went out with a halter to catch her, she would stand calmly and accept it. But, suddenly, and for no apparent reason, for the last couple weeks whenever i go out to the pen with a halter, she will take off at full tilt, and refuse to let me catch her. How can I teach her to let me catch her when I go out with a halter, so I can finally work with her?!

Somehow she has gotten a negative association with the halter. Are you the only one that handles her? Is it possible someone popped her w/a halter recently? I think first you need to reintroduce it to her in a non threatening way. Let her smell it, lip it, rub it all over her when she lets you. Give her a treat out of the same hand you're holding the halter in. Rub it on her while you're scratching her favorite itchy spot. You need to reteach her that the halter means only good things. Do you only go get her when it's time for her to work? Maybe go get her only to groom and pet her and let her back out.

This is whats happened to my friend & her mare too!
She was out in the paddock for two hours trying to get the darn thing on her.

My horse was somewhat like that, but with a lead shank.
Someone must of smacked him with it in the face & he was scared of it...
So, I would just go out & play with him for a bit & get his favorite treats & feed it with my hand that had a lead rope tied around me...
At first he bolted off but then he started getting confidence.
Another thing I did was put some leads by his water & food dish out in this paddock.
So he would notice it wouldnt do anything when he was eating or drinking.
Then I would let him see it,bite it,etc & then I would rub it on his face,shoulder,withers,rump-etc..
& I would do that for ten mins each day.
Now he has no problem with it

Well, yesterday I did start doing some of these things you have suggested, feeding her out of the hand holding the halter, letting her sniff it and lip it, and rubbing it on her face and neck. There is already some improvement, but shes still a little shy around the halter.

I'll go out there today and do what you suggested again, and try to get her used to it a little more. I'm not the only one who works with her, though I do most of the work. They people who own the place she's boarded at have a daughter who likes to go out and play with her, and even the owners of the stable go out on occasion and do a little work with her. That might be what happened, someone bopping her with a halter, because she was fine the last time I had it on her, and there was such a radical change....

Well, whatever the reason, I'm planning on going out there today and doing some of the thing you guys suggested

It may take a little time, but I'd be surprised if she didn't come around pretty quickly. Good Luck!

I agree with JeneJen, it might take a little time- took me about three weeks. But she'll soon see that you arent going to do anything bad with the halter. One other thing you might want to try is if she's okay with it, put the halter on her by her muzzle. If she lets you do that then rub it on her face & just talk sweetly to her

You might also want to have a little chat with the people boarding her. Let them know that you want her worked with gently and would appreciate it if they would ensure this.

There are a number of ways to approach it and if you are unable to get her to a round pen then doing what you are will work. I would also suggest that she goes back to a stall or a smaller pen where she is easier to work with and doesn't have a ton of room to run. The only thing I suggest is don't try to break a bad habit by creating a new one. It is okay to feed her treats but don't due it from your hand, find a little feed bucket to use. Otherwise you are going to get a mouthy horse especailly since she is a young horse. You don't want her constantly looking for treats on you and nibbling around on you. Crowding up in your space. It may be cute at first, but the first time she bites someone because she thinks they or you have a treat she could due some serious damage.

Good luck with her, and get her to a round pen for a little work, soon she will figure out that it is better to stand for the halter than run around.

My first suggestion is to leave the halter on. Use either a leather or breakaway[has a leather or velcro part] Especially @ a boarding facility, cross your fingers nobody forgets to close a gate=loose horse/no halter.
Next, the kid likes to play with her. How? By going in the field and squatting [predator stance-horses will run] or playing tag. Then what you have is a taught behavior.
Then, teach her[and bucket feed treats OK] It is easier to come to you or stand still.
How- Approach her as normal[ look past her not at her eye, and not fast or aggressive] If she stands, put your rope around her neck, then halter on. If she moves away, you send her @ trot or faster. Keep her moving for 2 minutes, tell her whoa, then try again. No luck, move her, 4 minutes this time. Contiue this process till caught, so not when you need to be somewhere else and are running late. Never get mean or angery, just make her do the work. The smaller the space the easier for you, but it will work anywhere. Hint, If she's out with others she will try to get to them, use them, herd-you-her. Depending on alot of variables may take once or quite a few times, but it will work. As the others suggested, once caught don't always work her, feed,pet,brush, etc...Horses aren't dumb and will associate being caught with work. She may easily be caught by someone else. Possibly whoever feeds, or @ certain times. Another option is don't bring her in. If I have a horse who isn't @ the gate when I'm ready to bring in, they don't come in. Funny, but the next day they are first @ the gate.
But you need to try to find out the other stuff also.

By play with her, I mean she will walk out and pet her, talk to her, etc. (Thankfully) she doesnt chase the horses around, she just likes to go and hang out with them.
I think(Ltc4h) I will try your suggestion for training her to accept the halter rather than run away, but it may have to wait a while. Right now she is at the friends house, and their horse-facilities include two large pastures, a tack room, and a shelter for them in each pasture for bad weather. aka-no small area, or place away from the buddies to work with her in. Soon (in a couple weeks or so) a place will be opening up for her at a stable where I know the owners, and thankfully they have a round pen. So, ya, it will have to wait, but as soon as I can, I will have to do some serious work with her to get her to accept the halter a little better.
For now though, she is calmer around it, and will let me near her with it. Still, sometimes she will take off, and that is what I'm needing the said training excercies for. But, hey, it will take as long as it takes, she will be my main summer project for this year, and so I'm not super pressed for time