What I did was just "gradually introducing it", as also recommended for other animals:
(Step 0: I made my dragon comfortable with being touched everywhere I would have to touch him when putting a harness on, and where the harness would be. So, dragon has no problem with touching his back, reaching under his belly and such.)
Step 1: I put the harness next to him and I touch it, play with it.
Step 2: I touch him with the harness.
Step 3: Harness is just put on his back (not wrapped around, not closed, it just lies there).
Step 4: Harness is put on, leash attached.
Each step only when he's absolutely relaxed with the previous one.
There are various kinds of harnesses; I have the one from Nectarlee that has a faux leather thing on the back and straps going around the belly. This way I don't have to pull it over my dragon's head.