I'll be doing this with a series or project I'm attempting to tackle over the next couple of years.

My plan is to give people an explanation as to why I'm interested in taking their photo.
I'd explain the project, the reason for the project, where they can get access to see the project once finished, and ask them whether or not they would like a print.

I think if people know why you want to photograph them they will be happy to participate.

Tom.