Even though bad reviews cannot be deleted, they can be responded to and corrected. This can turn a lemon into lemonade if subsequent visitors see that you have corrected whatever went wrong to the best of your ability.
Second, having a bad review makes your business look human. Most people realize that a business makes mistakes or can't please everyone all the time. Most understand that there are people that can never be satisfied no matter what.
That being said, my approach is to make getting reviews proactive. There are several sites that are predominately review sites. Instead of trying to get customers to put reviews up on all these key places, I think it is easier to focus on one. The one I zero in on is Kudzu. This is a quality site and is held in high regard by Google for reviews.
The best way to get a review is right when the customer has bought. Presumably, they are at the height of their satisfaction. Open up a lap top to the Kudzu review page and have them put in a review on the spot.
In the absence of being able to do that, Kudzu has a .pdf review form that can be customized for your business. Print out a bunch and have them ready right when the customer has bought. Hand them out.
Kudzu also gives people the opportunity to post a review on their web site. Alternatively, they have a banner/logo that you can put on your site and link it to their review page. Either way, in your follow up emails to your new customers (hint, hint) ask them for a review, tell them how to do it and provide the link.
Since reviews are so important to ranking on Google Places, putting one or more of these strategies in place should go a long way towards building reviews.