I hope this doesnt come across as too negative.. but do you only take him out to ride him?
I see this happen all the time.. horses living in a big pasture with a few to many buddies, and the only time they see people its because we want something from them. Therefore, the horse sees a person and runs because they dont want to be worked.
If that is the case, it could be as simple as just taking him out more to do stuff for -him- not for -you- =) So he'll see you coming with the halter and try to run, and when you finally catch him, take him out and let him eat some sweet feed while you brush him... or just walk around with him... or find a patch of grass and hang out with him while he eats... then put him back =)
I made a post similar to this topic on a thread about catching a shy rescue horse:
Im really big on being around my horses when I have nothing to ask of them =) A lot of people who have non-rescue horses make the mistake of only going out to their horses if they want something (to ride, to worm/give shots, etc)... Just go out and be around her. Horses are curious little things... More often than not, when i've worked with an abused/neglected/beaten/never handled horse and IGNORED it outright... they would come up to me with this look on their face as if to say: "Hey bub, everyone else wants to touch me, brush me, lead me, vaccinate me... why don't you? Am I not good enough for ya?!" LOL
I wouldn't go as far as to say never catch her unless she wants you to... because I personally think that is giving a 1,500 lb animal a little too much "control" over ya =) I agree with the concept of "dont go out and trudge up to her, and grab her to give her shots" but I think the best way to approach this whole situation is:
Try to be around the barn at the same time every day. She wants consistency in her life... if you start to become that consistent thing, she will automatically be drawn to you because horses really enjoy someone who is consistent with them... Neglected horses need consistency the most!
Hang out with her, get her interested in you without you asking for anything from her (not even to lead her).
When she accepts you being near her in her paddock/stall/pasture start very small by just touching her. If you see her begin to shy away briskly (but not startlingly so) pull your hand back and take two steps backwards (also briskly). Miakoda was increadibly hateful of people when I got her.. but she LOVED playing the mirror game with me.. She would lean to take a step back as i approached her, so i quickly took 2 steps back and before she ever lifted her foot to move away from me, she had actually taken two steps TOWARDS me.. Horses are so awesome like that, LOL!
Then once she enjoys your touch, and your company, if she is kept out with a halter on, just clip a leadrope to her and stand there. Don't ask her to lead, just stand with her and pet her. then unclip the leadrope and leave. Don't dottle, dont try to pet her afterwards, just go.
Once she enjoys the touch, company, and is tolerant to the lead, take her out to do something she wants to do.. perhaps you can hide carrots around the barn and have her find them (Skippy! loves that!) or, just find a big patch of grass and let her munch away! Always, ALWAYS put her away happy!! =) Because that's what she'll remember the next time ya come out to get her ^_^
If at all possible, I'd really recommend stopping the food bribing, because then she may just view you as a food dispenser. You don't want a horse to come to you because you have food... because they aren't really coming to you, just to the food :) I understand that at certain times we all have bribed our horses (lol!) But a horse will respect ya more if you don't use food to grab 'em. Charity was a feral, wild little thing when I got her and i spent 15 minutes staring her down in the corner of my field to catch her. I was SO CLOSE to getting some feed, but i resisted, and now i have a better horse because of it!
The whole process will take a while, at LEAST a week of constantly working at it... so be patient! As someone above said, if you rush it you will erase ALL your progress. This is also much easier to do with your own horse on your own land... because where she is at now, someone else might trudge into her stall and grab her which may undo all the work you had done (it all depends on the horse)
Same basic principle, only you're showing your horse you want to enjoy his company ^.^
The Mirror Game works really well, I can't boost it enough. Charity was completely wild when i first got her and she picked up on this. She wanted to be the one to blow me off, and when i would be the one to leave her, she got all sorts of flustered, lol!
Something else that really helps is when you are done riding, clean him off and spend time with him. Don't just put him right back in the field while the sweat is still moist on him. Because all he will remember is the ride and the work. Always put your horse away happy! =) After grazing him or spending time with him he should be in a good mood, and happy to see you next time.
As far as mounting from the ground, try what spastic dove said... That's how I got Joey and Skippy! to not wander on me when trying to mount.
My little saying is this: Your relationship with your horse is split 50 50. 50% of the time your horse is showing you he will take care of you when you're in the saddle, the other 50% is you showing the horse you will take care of him on the ground.
When that relationship gets out of whack, we end up with horses that are pushy, mean, and unruly... or scared, flighty, and grumpy.
Again, I hope i didn't come off as mean ^___^ i didn't mean to!