Pair Programming good practices from my experience

• Get the best hardware you can. Saving on hardware is stupid.
• One big screen is better than two. It is easier to look at the same thing. 
• Always talk out loud what you think. Your pair cannot read your mind.
• Switch roles frequently. At least every 25 minutes. It helps the pair stay focused.
• Take breaks. A day of pair programming can be exhausting!
• Use Test Driven Development. It helps you keep track of what your doing. It can improve the quality of your code. But do not forget the goal which is code that works, not test coverage.

Recommended readings:

• http://www.extremeprogramming.org/rules/pair.html
• http://guide.agilealliance.org/guide/pairing.html
• http://www.wikihow.com/Pair-Program
• http://blog.xebia.com/2010/05/09/practical-styles-of-pair-programming