When you are presenting your wares to a prospect include a page that simply lists four or five references for contact instead of your traditional glowing testimonial page. Urge your prospect to contact each for more information on how you or your product performs. (I've even gone as far as writing a list of suggested questions they might ask the reference - it helps them focus on benefits) In some cases your prospect may request this anyway.
What I have found is that when your current client is contacted for information (often by email these days) they will generally and immediately feel compelled to put in writing what amounts to a well-written testimonial. The key here is that, when approached by another business, they will write as they are speaking to a prospect. The copy will almost always be over the top selling you and in the perfect voice for you to re-use as a testimonial. (When a client writes a testimonial in the traditional way they often write is as though they are speaking to you. Many times this doesn't have the same marketing pop to it.)
Now here is where the fun part comes in. What I have also found is that quite often your existing clients will copy you on the communication they sent to the prospect. Bingo, instant testimonial, written exactly as you need it for your marketing materials.
Actually, using this strategy can be even stronger than just printing written testimonials as it involves your current clients in the active process of marketing and has the tendency to resell them on their decision to do business with you as well.
The only caution is that you spread the love around to as many of your clients as you can so that no one group of clients becomes burdened in the process.
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John Jantsch is a small business marketing coach and author of Referral Flood - How to generate a flood of new business without spending one dime on advertising. http://www.referralflood.com and Blog Lightning - How to create and promote your blog in a flash. http://www.bloglightning.com.