Document your code! Write the documentation before, during, and immediately after you write the source code: Write some documentation for each function and class before you write the code. This will help you understand its purpose and clarify its functionality. Finish the documentation while the code is still fresh in your mind. Otherwise you will have to study the code again when (if) you come back to write the documentation.
Place the documentation (including performance results, links to graphics, etc.) with the source code. The quality of the documentation is inversely proportional to its distance from the source code. I recommend using Doxygen to build reference manuals in HTML or PDF format.
If you have multiple packages in a project it may be best to have separate Doxygen configuration files for each package and generate the documentation for each package separately. This provides a logical grouping of the items. You can get cross-links between the packages by using tag files. The list of tag files should include the packages that are used by the target. Typically do not include packages that use the target in the list; doing so may inject distracting template specializations into the inheritance graphs and collaboration graphs.
Use unit tests to improve the reliability of your code. For each source file, have a unit test file. Test each non-trivial piece of functionality. Be sure to consider special cases like negative values or null pointers. It is important to be able to easily use your unit tests. You should be able to compile and/or run all of your unit tests with a single command. This makes it easy to tell if an improvement in one package breaks functionality in another.