Right-click on the divider (between applications and shortcuts to folders) on the Dock and selecting "Turn Hiding On"
System Preferences > Dock > Automatically hide and show the Dock
These steps can be found via "mac hiding dock" as search terms in a popular www search engine, such as this
The Menu Bar, though it is only 22 pixel high, thus I hardly see any additional gain, but I use a 1050 pixel and 1200 pixel high display, can be hidden via these steps:
1. Macintosh HD
4. Core services
5. Right click on Finder
6. Show Package Contents
8. Drag the Info.plist to your desktop (Dont forget to save another one for backup)
9. Open one of the the Info.plist and click Add Child
10. Choose Application UI Presentation Mode and then next to it type 4.
11. Save it
12. Then Drag the Moddified Info.plist to your Contents and replace it with the existing one.
13. Relaunch Finder or restart your Mac
14. You are done !
also found via a search engine and using "mac hide menu bar" as search terms, though I don't know, if it works with Lion.