This is a very common issue, from what I hear. Here are five solutions from [http://frugalliving.about.com/od/colthing/qt/Line_Dry_Stiff.htm|about.com]:
# Add a cup of white vinegar to your wash cycle to help dissolve the laundry detergent.
# Cut the amount of detergent that you use. Half the recommended amount is usually plenty.
# Run your clothes in the dryer for 10 minutes, before putting them out on the line.
# Shake out your clothes before hanging them.
# Hang your clothes out on a windy day.