I recently purchased a SSD drive and was looking to optimize some of the games I play on a regular basis. The issue with steam is that it centralizes everything to one main folder. This of course does not put you in the dark for getting faster loading out of your $$$ SSD investment. If you are running Windows 7 you will want to do the following.

Copy the game you wish to move from the original folder in Steam
Example: C:\Program Files\Steam\steamapps\commom\GAMENAME

Copy it to the new location you are looking to play it from
Example: S:\MyGames\Steam\GAMENAME

After the folder has completed copying, in terms of testing, rename the GAMENAME folder to _GAMENAME This will pretty much treat the original folder as if it is missing. You can then delete the folder after you have tested the link.

Open a command prompt and you will type the following.
mklink /J "C:\Program Files\Steam\steamapps\commom\GAMENAME" "S:\MyGames\Steam\GAMENAME"

You should see a statement telling you a link was created. Look at the main common folder and you should now see a shortcut folder created. If you double click that folder it will automatically bring you to the new location where you copied the game.
Open up steam and run the game to confirm it is working properly. After you have confirmed it, you can then delete the _GAMENAME renamed folder to reclaim space on your drive. If at anypoint you want to go back to the way things were. Simply delete the shortcut link folder in the original steamapps/common folder and copy the folder from the location you had copied it to. It will be like nothing ever happened.

Enjoy!