Some emerging research suggests that mixing sunscreen and insect repellent on your skin at the same time may cause your body to absorb more of the DEET in most repellents. To play it safe, keep bugs away by spraying repellent on clothes rather than mixing with sunscreen on your skin, says Dr. David Schlessinger, medical director of Schlessinger Eye & Face clinic of Woodbury, New York. Test a small area of the fabric first to make sure it doesn't stain. 

-Better Homes and Gardens, May 2009