In my experience (sales at a large company), I would always have to write stuff like this myself. Customers were generally reluctant to write something not because they didn't want to, but because they didn't have the time or motivation to sit down and write it.
Whatever you write should flatter your customer as much (or more) than you. This will help your customer see the benefit of using their name in your promotional materials. It also benefits you--there must be a reason this really awesome-sounding customer is using your product. (think SAP ads)
Ghost-writing a testimonial that flatters your customer changes the whole proposition for them since they just have to say "yes" as opposed to agreeing to spend an hour or so writing something that they don't think will benefit them.
In some cases they might get really into it after seeing your "sample" and provide a bunch of edits or other ideas. Again, it's always easier to comment on someone else's work than create your own. So make it as easy as possible for them.
First your product has to be good enough that people will say nice things about it.
Then when they send you an email which says something nice, ask them if they would allow you to use a specific quote from it as a testimonial on your website.
I have never had a person refuse that request. They wrote it because they meant it and they don't mind others hearing it as well.
User provided testimonials are amazing. They say things about your product that you would never have thought of saying yourself. They often give superlatives, which would be embarassing for you to say yourself, but in the words of someone else, sounds terrific.