There are, of course, some standard areas that should be covered by your resume—a chronological employment history, your education, a matrix of your technical skills, a statement of the type of role you are seeking. Above and beyond those items, these are the things that you will want to ensure get highlighted when it comes to periodically updating your resume.
Certifications and Classes: Both of these look good because they show that you are willing to take the time from your busy schedule to keep your skills up to date.
A Link to your Technical Blog: Many bonus points are given for someone who enjoys their craft so much that they blog about it. If you can also highlight personal software development projects, all the better.
Local User Group Attendance: Again, taking the time out of your busy schedule to attend a User Group meeting shows a commitment to keeping your skills and knowledge up to date.
Speaking at Local User Groups or Tech Conferences: This shows a commitment to the developer community and displays a likely higher level of responsibility in a prospective employee.
Contributing to the Larger Software Development Community: Answering technical questions on boards such as StackOverflow or contributing to open source projects.
Instances of Revenue Generation, Cost Savings, or Implementing Efficiencies: From a business perspective, these are the "big three" that are at the forefront of every manager's mind since they directly impact the overall profits of a company. Did a piece of code you wrote result in revenue generation? Note that down as a specific accomplishment for your current job! If you implemented a network monitoring tool that resulted in less down-time, highlight that (including an actual cost-savings figure makes it even more impressive!).
These are all things that will make your resume stand out from the other resumes—but just as easy as it is to note them down periodically, it is just as easy to postpone until they've been forgotten and lost. Capture these gold nuggets while they are fresh in your mind and maximize your chances of getting hired.