Anytime you can use the keyboard instead of the mouse, you are likely to be faster and more efficient.  When it comes to SSMS, these are a few of my favorite things…

• Ctrl + K + O to switch connections in a window
• Shift + Alt + Click to block select and move things around easily

• Select a block of code, then Ctrl + K + C to comment it out (Ctrl + K + U to uncomment)
• Ctrl + O to quickly open a file
• Ctrl + M to turn on execution plans (Ctrl + L for estimated execution plans)
• Alt + F4 to close SSMS
• F7 for Object Explorer Details
• Alt + F + A for Save File As
• Ctrl + F4 to close current query window (Ctrl + N to open a new one)
• Ctrl + A + K + Y to quickly and neatly format code (if using SQL Prompt)
• Ctrl + R to show/hide query results
• Ctrl + G to go to a particular line of code

If you have any to share, please feel free to put them in the comments!