Two possibilities, already kind of referenced by the other answers:
- Make sure you are using the Debug build of the assembly instead of
the Release build, because the Release build will remove or optimize
your code. - Make sure you are updating the version each time you
deploy the assemblies in Visual Studio (on project properties tab).
When you increment the version, CRM will be sure to unload the old
assembly version and reload the new one without an IIS reset.