Mug shots. When unsolicited testimonials come in (the best kind) get back to them and ask if they have a photo you can use. The same applies if you ask for a testimonial, ask for a photo as well. In this day and age it's really not difficult to get a photo scanned and converted to a JPEG. They can then email it to you. If this is a problem for them ask them to mail one in so you can scan it in yourself. Be sure to always post it back to them with a nice note of thanks. Both your website and your prospects willingness to buy will be enhanced by this simple technique.
Add A Signature. I'm always disappointed when I get to the end of a testimonial, only to find there is a simple text signature. OK, I can understand that some people might be reluctant to let their signature be openly seen in the public domain, but it is well worth asking for a scanned JPEG or an original by post, in the same way as for photos (above). A signature adds to the feeling that here is a real person who is serious about using the product and will put your prospect more at ease.
Video Clips. Let's face it; video clips are THE thing on the web these days. Everybody's doing it. If your testimonial giver has a webcam ask them to do a brief video of around 30 to 60 seconds and email it to you. To find out more about putting video onto your site search for something like 'real video' or 'website video'.
Audio Clips. If video clips are really out of the question, then try audio clips. These are much easier to do and can be recorded over the phone. Just use a mini tape recorder, but make sure your subject knows you're doing it, and play the finished result to them before going live.
Contact Link. Ask your testimony giver for some contact information to go under their testimonial. This will give prospective buyers the opportunity to visit their website and maybe ask them questions. Put this under their photo or at the end of the testimonial. This adds another layer of trust in your prospects mind. All good for sales!
One last point: It's probably best if you can show a mixture of these tactics on your site rather than using all the same type. Not all your visitors will want to listen to a whole load of, say, audio clips, and will welcome a choice of format.