If you have your security preferences set to require a password after sleep or screen saver begins (System Preferences > Security) then you can use Control-Shift-Eject to turn sleep the display. It won't start the screen saver, but the display will go dark and you will need to enter your password to wake it up.
Another option would be to use Automator to create a service to start the screen saver and set a keyboard shortcut to turn it on.
To do this, open Automator and choose Service as the template for a new workflow. Type "screen" in the search field to locate the "Start Screen Saver" action and drag it to the workflow. Set the "Service receives selected" option to "no input". Save your workflow with the same name you want it to have in the Services menu.
Next, open System Preferences > Keyboard and go to the Keyboard Shortcuts tab. In the left column, click on Services and scroll the right column down to the bottom to find the Service you just created (it will be listed under the General category). Then assign whatever keyboard shortcut you'd like to use.
Thanks Seph. Putting the displays to sleep works for me.