Microsoft has a nice KB article posted describing how to re-arm activation for Windows Server 2008. You can re-arm Windows Server 2008 to provide up to 240 days of usage without being required to activate the server. One example of when you may need to do this is when you have lab/dev servers that you don’t want to waste a MAK activation key. This feature also allows you to extend Eval copies of Windows Server 2008 beyond the initial grace period. Re-arming the activation is fairly simple. Just open a command prompt and type slmgr.vbs –rearm
You will then need to reboot.
After the reboot your grace period should be reset to another 180 days. The full kb article for this process can be found on the Microsoft Help and Support site at the source link below. The KB article also includes details on how to automate the rearm process.