I work for a company that makes powder coating and have had a few coaters refurb frames for me.
Pointers:
If you have seen there work and are happy, ask for the colours they work with and leave every thing else up to them.
You should look at edge lines where they have protected a surface so it isn't coated and the coverage on external corners and internal corners. You should not be able to see the substrate on the former or a build up on the later. If you do it either poor application or poor setup either way the job won't be as good as should be.
If you have concerns over "masking" ask for the frame back after it has been stripped. This is a double edged sword as you will be happy with the masking but the coater will take no responsibility for the job as he doesn't know what you have done to the surface of the frame.
If you do mask off, WEAR GLOVES and don't let anyone touch the frame, grease does affect the charge and powder transfer.
Use Silicon rubber bungs ( Ask your coater if he has some you can use) they are way easier to use than set screws and leave a far neater cut line to work to.
Use aluminum tape get the proper 3m version as the cheapy ones leave loads of residue.
Ask which way they want the frame hung up and make sure you leave a big olde loop of wire or 2 so they can just stick it straight on their rail with out dickin' about having to make hanging loops.
IF IT GOES WRONG, tell them as most coaters will try to rectify issues.
My Preston that i recently coated - I did ask for this back after blasting so i could mask it of.