Avery is one, and I've had her for nearly 10 of those 12 months. I purchased a Flight Harness for her 8 months ago, and we're just getting comfortable with it going over her head. When she first saw it, she absolutely flipped! It terrified her.
So, after lots of patience and continuous exposure and positive reinforcement, here we are today! Some birds immediately take to the harness, but are often birds that like being handled. I do personally believe that once a parrot recognizes that flight harness/flight suit = flying outside, they really begin to tolerate it and appreciate it because of the association. Every time Avery is exposed to her harness, she knows she gets treats and lots of love, so I'm hoping once it's on all the way she'll love it even more. Here's my take:
1.) Positive exposure - every time your bird sees the harness, it needs to be POSITIVE. Treats, happy voices, lots of love, favourite foods, etc. This is often slow, because birds are afraid of the unknown. Just try to make sure you don't allow the birds to chew on the harness during exposure, otherwise they may learn that chewing it is okay. Don't underestimate the chewing power of any parrot. Eventually she let me lay it over her back, and every time she did I would give her lots of kisses and a great reward.
2.) Comfort in touching - your birds need to be okay with you touching their wings, touching their head, and closing their eyes. I began training with my hand and a hair band. With my hands I would touch Avery's wings for three seconds (and count out loud) and give her a reward after she tolerated it. If she didn't, I wouldn't force it but I wouldn't reward her either. I started lightly lifting her wings, and the longer she tolerated it the bigger and better the treat. As for her eyes/head, that's where the headband came in. I would ask her to touch her chopstick for target training through the hairband. Started far away and moved closer and closer until eventually she had to put her head all the way through. After she was comfortable with that, I would "drop" it on her and reward her when she tolerated it. Now we can do that with the harness.
I NEVER force it on her, ever. I don't want to damage our bond by MY desire to take her outside. Avery doesn't understand that I want her comfortable with it so she can fly, and she never will until we get to to that point. It may take a few months, it may take years. Every parrot is different.