Force Directory Synchronization

To force synchronization, log into your server that has the Directory Synchronization tool installed.

Updated in 2014 to the following:

1. On the computer that is running the Directory Sync tool, start PowerShell, type Import-Module DirSync, and then press ENTER.
2. Type Start-OnlineCoexistenceSync, and then press ENTER.

Second Option run:
C:\Program Files\Microsoft Azure AD Sync\Bin\DirectorySyncClientCmd.exe initial
C:\Program Files\Microsoft Azure AD Sync\Bin\DirectorySyncClientCmd.exe delta

Old method

Open up PowerShell and navigate to
C:\Program Files\Microsoft Online Directory Sync

Then start the Directory Sync Configuration Shell by typing
.DirSyncConfigShell.psc1

Then run the following cmdlet:
Start-OnlineCoexistenceSync