Hi DJ Nostalgia (great name).
One thing I’ve seen in the world of wedding DJs, for example, is that they, or someone the bring with them, record a short video of the couple late or after the wedding in which the couple speaks briefly about the music you played, the vibe you generated, the fun they and their guests had, and (if you do this) your job as an MC, too.
These then videos are posted on the DJs website, YouTube channel, Facebook DJ Page, and Twitter, and often also on forums and websites that focus on that specific clientele.
(BTW, as someone with a documentary film production background, I would advice you to include in your contract a clause called a “release” in doc filmmaking in which the client authorizes you to take photos and videos during the event and granting you the rights to use these in your marketing materials and online platforms.)
Regardless, I think that developing a good human interaction and bond with the client before and during the gig is essential for the client to recommend you to others and also feel comfortable with writing review.
Finally, consider also sending him\her\them a thank-you email a day or two (no more) after the gig describing very briefly how fun it was to perform for him\her\them.
Anyway, hopefully other mobile DJs will chime in here with more advice.
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