In one of my previous jobs we had a changeover in Quality managers. The new manager decided after some time, while we were preparing for FFSC22000, to change some of the GMP's due to changing our food safety certification.
He proposed the changes to the GMP's, which were minor, and the reasons that the changes had to happen. Some of these things were beard nets covering the whole mouth up to the mustache, hairnets completely covering the ears, etc. We had used hairnets and beard nets for years but we had a great deal of push back.
The first thing you must have is manager commitment. If you want to institute an across the board policy on hairnets you need to make sure that all of management is going to follow the rule too. Have them start to wear hairnets in that area before you even tell the employees. Make it obvious to the employees by walking in to that area with a hairnet on without going into any other area (if you can).
Some push back will happen no matter what approach you take.
First explain to your employees why there needs to be a change. If you have a food safety program in place try to find some verbage out of it to back up your position.
Ask them to help you by coming on board with the team (Not sure if you have a HACCP team or Food Safety Team... but sometimes just saying Team works if you have a small company)
Afterwards go in and check up on it. Constant checking will be required. Carry a hairnet with you and hand it to them. I used to point at my chin if they needed a beard net on and they weren't wearing one. If they aren't wearing what they are required to wear when you walk around talk to them about it. Ask them if there is anything they don't understand or that you need to clarify.
No matter what don't waver in it. Don't let people just walk past you without it on others will see that and figure they won't need to either.
Personally I like being Draconian about it. Things I have said are "Those are the rules. If you don't like it the door is that way (points to the door)." However when we changed the rule to employees not being allowed to wear their outside hats in the plant anymore I was able to explain to a guy that I knew hunted why it was the new policy "How can we know you didn't go hunting with that hat on and get blood on it? That would be a food safety hazard. How do I know you didn't go mudding in a cow pasture in your 4 wheeler and get something on the hat?"
So you can get some flies with honey... some with a fly swatter. But between the two types you should get them all over time.
But it all comes back to manager commitment. That's why it's normally the first thing in a food safety standard. If you can't get the commitment of management to follow the food safety rules and punish those who break them then there is no point in even trying.