The best way to get a good decorator is through recommendation and seeing their work for yourself before contacting them.
Being a member of an association does offer customers some assurance but they are not strictly regulated, like gasfitters for example. I'm not a member of any organisation, but have often had to tidy up work done by decorators who are. That's not to say all the members do poor work of course, but there are some that do.
If you insist on getting a decorator who is a member of one, then try to get them from a non-profit organisation and not someone who is likely to try and sell you the product of the association they are affiliated with.
Questions you should be asking include:
Are they qualified? (Not essential, but recommended.)
Can they provide references and examples of their work?
Do they have public liability insurance?
Is the work guaranteed, and for how long?
What products will they be using? (If they supply materials.)
Can you have a fixed quote in writing?
How do they expect to be paid? (On completion is usual and cash up front a no-no!)
There are lots of things to ask really, but these are the basics which I hope help you.
