In addition to maximizing the recommendation request, there are also some simple steps to maximizing the rate of those requests turing into recommendations.
1) Once you have completed the service, give your customer a heads up that they will get an email with the request and let them know a recommendation is very appreciated.
2) The closer to the service when you sent them the email, the higher rates of response. We are seeing on average a 60% response for service provided recently, but that percentage lowers the longer you wait to ask for a recommendation.
3) If they are not expecting the email for the request, a quick phone call with a voicemail giving them the heads up on the email is usually increases the chances that they'll give you a recommendation.