Check out this great article by Marion Boddy-Evans, at About.com.
Plein Air Painting: Taking Your Paints Outside; Practical tips for plein air painting, or painting on location.
Some common questions like these are anwsered...
What and Where Do I Paint Plein Air?
How to Deal With Spectators While Painting Plein Air
How to Cope with Changing Light When Painting Plein Air
Do I Have to Finish the Painting Outdoors?
What Art Materials Do I Need to Take With Me for Plein Air Painting?
Is It Safe to Take My Paints on a Plane?
Do I Need an Easel for Plein Air Painting?
How Do I Transport Wet Canvases?