In order for a CRM implementation to be successful, you’ve got to be able trust the data. Here are some rules for maintaining data quality.

Standardized Naming Conventions
Before you implement Salesforce it’s a good idea to come up with standardized naming conventions for company names and addresses. For example is it U.S.A., USA, or US? Is it Acme or Acme LLC? Here is presentation which lists best practices for standardizing naming conventions .

Divisions and Subsidiaries
When working with large enterprises there is often a web of divisions and subsidiaries which can be difficult to navigate and often results in duplicate accounts. If this is a challenge your organization faces, you might consider Hoovers, Dow Jones SalesWorks, and TrueAdvantage.  This will help you pull in accurate information based and map out complex corporate families.
    
Duplicate Leads and Contacts
Duplicates can come from any number of sources including your website, lists, or they might even be manually entered by reps. There are several Salesforce Partners who do an excellent job of tackling this problem, among them are Ringlead, DemandTools and Trillium Diamond Data all of them have received rave reviews amongst customers. It’s also important that you train your reps on how best to search for duplicates. While this is different for every business, it’s often effective to search against the tail end of the email address, for example acme.com. This will catch both the contact you were looking for as well as other contacts at that same company. 

Data Quality Dashboards
With Salesforce you can set up what is called a data quality dashboard to highlight when fields are left blank. For example you might have a dashboard that indicates the number of times main competitor was left blank. You might also track overdue tasks or last time the record was updated.

Free AppExchange Apps
There are a bunch of free tools on the AppExchange which can be used to improve data quality. The most widely used is probably Ron Hess’s Excel Connector. This enables you to extract data from Salesforce using Exel, perform updates and push it back. Ron has also built a number of simple components for performing bulk tasks like mass update opportunity dates or mass update contacts addresses. 

Downloads

• Data Management: Getting It In, Getting It Out, Keeping it Clean  (Breeze Presentation)
• Improving Data Quality: Best Practices for Keeping Data Clean and Relevant  (Breeze Presentation)