New / Modified artifacts should be reconciled with the Source Control database on a periodic basis - Checkpoints.

In a team environment it is recommended that code that has been modified be checked in as frequently as possible, and only in a stage where the code is working. Checking in code that has a compilation error, or logical incompleteness that can affect application build ability, functionality or stability is NOT recommended, especially if multiple developers are working with the same artifacts in the repository.

For those specific modules that need to ensure that no parallel changes (branching of release) is being done, it is best to exclusively check out the artifacts and leave a note that indicates the reason for doing so, in order to communicate to other team members / developers the reason for the exclusive check-out.

There are multiple source control platforms out there, PVCS, CVCS, CVS, Subversion, Bazaar, Microsoft Visual SourceSafe, Microsoft Team Foundation Server, Rational Clearcase, PerForce, etc. Irrespective of platform, it is essential to have a user-guide to your specific platform (more like a cheat sheet - remember, quality not quantity) which you can share with developers to enable them to effectively use the repository.

Furthermore, it is extremely desirable to incorporate role-based security into your source control repository, by integrating with roles and users from Active Directory or LDAP / Samba to ensure that you dont spend half your time administering users.

Subversion is one of my favorites and integrates well with Apache's mod_ldap to provide an excellent Source Control solution, which is both time-tested and $CHEAP$. Integrates with Active Directory as long as your domain controller allowes anonymous LDAP queries, which is the default configuration.

With VisualSVN Server and TortoiseSVN for Windows platforms, implementing Source Code Management is easy on a tight budget. Both these tools are Open Source. For companies with a Budget, they can purchase VisualSVN Server Enterprise, or even go to a more Advanced platform like Microsoft Team Foundation Server.