How can I lock screen on lxde

You should be able to bind the screensaver lock command to CTRL+ALT+L by editing your Lubuntu keyboard & mouse configuration file

Add the following to ~/.config/openbox/lubuntu-rc.xml

<keybind key="C-A-L">      
  <action name="Execute">        
    <command>xscreensaver-command -lock</command>      
  </action>    
</keybind>

This should be located inbetween the XML tags

<keyboard>
...
</keyboard

Logout and login for the changes to take effect.

I would elaborate/combine some of the answers. First (in a terminal) start your editor:

vi ~/.config/openbox/lubuntu-rc.xml

Then search for the mentioned faulty code:

<keybind key="C-A-L">      
  <action name="Execute">        
    <command>xscreensaver-command -lock</command>      
  </action>    
</keybind>

and change it to use the dm-tool:

<keybind key="C-A-L">      
  <action name="Execute">        
    <command>dm-tool lock</command>      
  </action>    
</keybind>

Then add a section, so the Windows+L combination works too:

<keybind key="W-L">
  <action name="Execute">        
    <command>dm-tool lock</command>      
  </action>    
</keybind>

Finally, finish the editor (saving the file) and activate it:

openbox --reconfigure

Good luck and thanks for all the answers found here...

We can also use "lxlock" command that's provided by the lxde. I just found it on Ubuntu 14.04. We can add the below lines in our openbox config file in the keyboard. vim $HOME/.config/openbox/lubuntu-rc.xml

<!-- keybinding for Screen Lock-->
    <keybind key="W-L">
        <action name="Execute">
          <command>lxlock</command>
        </action>

And then restart the openbox with the following command.

openbox --restart

This will enable locking of the desktop by hitting Windows Key + L