As noted above, the CC has a set of possible assurance requirements that can be selected, and several predefined sets of assurance requirements (EAL levels 1 through 7). Again, if you're actually going to go through a CC evaluation, you should examine the CC documents; I'll skip describing the measures involving reviewing official CC documents (evaluating PPs and STs). Here are some assurance measures that can increase the confidence others have in your software:
Configuration management (ACM). At least, have unique a version identifier for each TOE release, so that users will know what they have. You gain more assurance if you have good automated tools to control your software, and have separate version identifiers for each piece (typical CM tools like CVS can do this, although CVS doesn't record changes as atomic changes which is a weakness of it). The more that's under configuration management, the better; don't just control your code, but also control documentation, track all problem reports (especially security-related ones), and all development tools.
Delivery and operation (ADO). Your delivery mechanism should ideally let users detect unauthorized modifications to prevent someone else masquerading as the developer, and even better, prevent modification in the first place. You should provide documentation on how to securely install, generate, and start-up the TOE, possibly generating a log describing how the TOE was generated.
Development (ADV). These CC requirements deal with documentation describing the TOE implementation, and that they need to be consistent between each other (e.g., the information in the ST, functional specification, high-level design, low-level design, and code, as well as any models of the security policy).
Guidance documents (AGD). Users and administrators of your product will probably need some sort of guidance to help them use it correctly. It doesn't need to be on paper; on-line help and "wizards" can help too. The guidance should include warnings about actions that may be a problem in a secure environemnt, and describe how to use the system securely.
Life-cycle support (ALC). This includes development security (securing the systems being used for development, including physical security), a flaw remediation process (to track and correct all security flaws), and selecting development tools wisely.
Tests (ATE). Simply testing can help, but remember that you need to test the security functions and not just general functions. You should check if something is set to permit, it's permitted, and if it's forbidden, it is no longer permitted. Of course, there may be clever ways to subvert this, which is what vulnerability assessment is all about (described next).
Vulnerability Assessment (AVA). Doing a vulnerability analysis is useful, where someone pretends to be an attacker and tries to find vulnerabilities in the product using the available information, including documentation (look for "don't do X" statements and see if an attacker could exploit them) and publicly known past vulnerabilities of this or similar products. This book describes various ways of countering known vulnerabilities of previous products to problems such as replay attacks (where known-good information is stored and retransmitted), buffer overflow attacks, race conditions, and other issues that the rest of this book describes. The user and administrator guidance documents should be examined to ensure that misleading, unreasonable, or conflicting guidance is removed, and that secrity procedures for all modes of operation have been addressed. Specialized systems may need to worry about covert channels; read the CC if you wish to learn more about covert channels.
Maintenance of assurance (AMA). If you're not going through a CC evaluation, you don't need a formal AMA process, but all software undergoes change. What is your process to give all your users strong confidence that future changes to your software will not create new vulnerabilities? For example, you could establish a process where multiple people review any proposed changes.