The PCI Security Council supplies a document to merchants that provides a Prioritized Approach to PCI compliance. This document is quite good because it breaks down the standards into 6 milestones - what to do first, what to do second, etc. according to what will have the biggest impact in safeguarding your customer data.
Following the document and this index should help you address that most important PCI implementation standards quickly.
Source: PCI Prioritized Approach