I find it's all in the approach. I am saying this as all three of the horses I have owned have been hard to catch.
How are you approaching the horse? Never walk up to them dead on or look it in the eye as they read your intent to catch them. Most will turn and walk away as soon as they see you coming.
Walk around the horse - do not approach the neck or head. I like to walk passed a horse. If I am approaching from it's left, I want to walk behind it until I am on it's right side. It will usually watch me, but won't run. Stop, look at the horse, and walk away. You now have the horse's curiosity ("Why did she not halter me like always?").
Walk back to the horse, again, not approaching the head or neck. Get to the flank if you can and give the horse a pet - then walk away. Repeat this until you can touch the horse's neck, and preferably, touch the horse all over without it moving away. Some people are going to seriously detest me for this, but I use treats. Once my horse allows me to rub it everywhere, she gets a treat. This keeps her interest from waning and she realizes she gets a reward when she doesn't run from me. I keep the treats in my pocket so 40 horses aren't chasing me all over the pasture - much easier than handling a grain bucket! Also, if this is your first time using treats you can bring a treat bag to shake. It stays closed, but makes noise - your horse will likely associate it with food, as it already does so with the grain bucket.
Repeat the same process on the other side of the horse. Once the horse is relaxed with you petting it, put the lead rope around its neck. The majority of horse's know they are caught once the lead rope is around them.
Pet the horse some more and, with the lead rope still on the neck, halter your horse.
If you have the time (I suggest you practice this when you have a few hours to spare), walk the horse around the pasture, spending some time with it. Take the halter off and release the hose. Give a treat if you like to do this, then walk away from the horse. Rinse and repeat.
Unfortunately, with horse's like this it's a repetition process and usually involves you catching and releasing multiple times over the course of a few weeks. Also, when you go out to visit your horse, don't always put them to work. Catch them and take them for a nice groom or bath or something else they enjoy. Then put them back in the pen. Make it look like being caught doesn't always mean unpleasant things and that you are a joy to be around.
Also, you can use friendly horses in the pasture to your advantage. Go up to another horse and pet them and talk to them. Your horse will watch and see that you are not a threat to them. Keep an eye on your horse - every time you move, you should have your horse's attention. If not, you are doing something wrong. Walk around, pet other horses, etc until it is watching you with interest.
Hope this helps! If you are not keen on feeding treats then ignore the whole bit about them