Hold down SHIFT during normal boot, until the loading bar appears. It will take longer than usual to start the system. Some services are not available.
Safe Mode will clear several caches and verify the startup disks.
From the command line, or on remote systems, boot into Safe Mode using this:
sudo nvram boot-args="-x"
When you want to boot into “normal” mode again, change the startup parameters to nothing:
sudo nvram boot-args=""
Very handy article from the Apple Knowledge Base: