These steps are no longer needed as Steam now supports Optimus. Click here for information on Steam's implementation.

Most games can be started under Optimus easily. This does not include Source games. If you are looking for instructions for Source games, check here.

First, enter the directory where the game's binaries are located.
Next, many games (especially ones with DLC) require you to define an app ID. For more information on this, click here. Finally, start the game with the following command:
(Replace 'game-binary' with the name of the actual binary.)

List of known games that support this method:
(Leave a reply if you know of a game that is not on this list.)

• Killing Floor
  
• World of Goo
  
• Psychonauts
  
• Amnesia
  
• Serious Sam 3
  
• Superbrothers: Sword & Sworcery EP
  
• Crusader Kings 2

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For some people Steam will start using the Intel card perfectly fine. For others, Steam crashes instantly. If you are someone who is unable to start Steam without using optirun, try the following command:
NOTE: If you wish to start Steam with optirun after this you must delete the previously linked file. You can do that with the following command:

The above error will often show up when you launch a game outside of Steam. It can cause problems with multiplayer, DLC, etc.

To fix (in terminal):

Note: Replace '/path/to/binary/' with the actual path where you launch the game. Also replace '00000' with the ID of the game you are trying to play. To find the ID, use one of the below websites:
http://marlamin.com/cdr/
http://cdr.thebronasium.com/