How to lock screen on MacBook Air?

I know this has been asked many times (Is there a similarly fast alternative to Crtl + Shift + Eject for new Macbook that lack eject keys?) but the hotkey combination does not work on my MacBook Air that has no eject button running 10.7.

So, what hotkey combination can I use to lock the screen?

Edit: pressing the power key on a MacBook Air while holding control and shift does not work in 10.7. The power key can only be substituted for eject in 10.8 and later versions of OS X.

Putting displays to sleep only locks the screen if this setting is enabled:

You can also put displays to sleep in 10.9 by running pmset displaysleepnow.

Another way to lock the screen is to enable the Keychain menu extra from the preferences of Keychain Access and then select Lock Screen from the menu extra:

An alternative way to lock the screen is to log out to the fast user switching screen. You can do it by running

/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend

or by using the lock action in Alfred:

MacBook Air replaces eject button with power button. Use this instead:

Update 2013-11-23: I apologize for not reading the question carefully. I missed the part where you want to actually lock the screen. The combination key below only turn off the screen without actually locking it.

Control+Shift+Power/Eject Button

If you want to enter sleep mode (where you also lock the screen), the answer provided above is correct. The combination key to enter sleep mode is:

Option+Command+Power/Eject Button

Here's something for anyone who, like me, scoured the web for a simple Mac equivalent to "Windows-L" to lock the screen, and found that all the solutions either required third-party software you don't want or a special key that doesn't exist on your (generic external) keyboard. The following worked for me in Mavericks (10.9.3)

1. Launch the "Automator" application and create a new document of type "Service"
2. Specify that the service receives "no input" in "any application"
3. Add the "Run Shell Script" action to the service
4. Paste the following into the "Run Shell Script" action's text area: /System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine
5. Save the service as "Start ScreenSaver" and quit Automator
6. Launch the "System Preferences" application and go to the "Security" preference pane
7. Under the "General" tab, enable "Require password [immediately] after sleep or screen saver begins"
8. Now go to the "Keyboard" preference pane, "Shortcuts" tab, "Services" list item, and find your "Start ScreenSaver" service under "General" near the bottom of the list on the right, and double-click it.
9. Press your chosen keyboard shortcut (I like control + option + command + L, so I can just mash all three modifier keys together and hit L for lock)

EDIT: I had trouble after an upgrade to OSX El Capitan. I was using the more-succinct AppleScript content:

activate application "ScreenSaverEngine"

which I thought should have been more future-proof than the shell-script incantation of step 4, but the first time I used it, it claimed to be unable to find ScreenSaverEngine. This was fixed after I started ScreenSaverEngine.app once explicitly from the command-line:

open /System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app