People don't tend to talk in perfect, quotable sentences. If a client has complimented your work, ask if you can quote them, and send them an email confirming what the quote will say.
Testimonials have more power if they can be credited to someone and include a little detail about the job and location. Most clients won't want to share their full name, but a little extra detail ("John, London, four-bedroom house") can make a big difference.
Testimonials are much stronger if they come with pictures. Be sure to ask for permission to use images of your finished work, especially if you plan to send those images to new clients. And make sure you remove anything that could identify the owner or the address - such as family photos or mail. The Showcasing tool is a great way to show off your work.
Make sure your full contact details are on everything you share with clients, including business cards, invoices and emails, so your clients know where to send feedback. These details should include all your social media information too, so clients can tag you on their social channels.