It sounds obvious, but it's often overlooked. The internet is overflowing with review sites, and although most of them are about either hotels or online shops, I'm noticing more and more 'bricks and mortar' businesses are getting reviews. The wide spread adoption of Google places will certainly be contributing to this, and with its prominence in google searches, online reviews by your customers are only going to become more widespread.

This can work for and against you. Clearly you need to be aware of the possibility, in order to respond where appropriate to any negative reviews. One of the best ways to do this is to setup Google alerts. This allows you to be emailed, either immediately, or daily when your search phrase (for example, your company name) appears in a new google search.