How can I prevent a swollen labia after foreplay?

Determine cause. First determine cause. If due to excessive force or friction or prolonged play, then be more gentle, use lubricants or decrease the amount of time. However it may be due to latex or other sensitivity to products used. Applying ice afterwards may help. Discuss with your gynecologist.
Depends on cause. The labiae can become swollen with sexual excitement, but they can also become more so by friction, or infection. Try some lubrication and keep things clean as well.
Increased lubricatio. Swollen labia are often caused by excessive friction which can be reduced with water based lubricants.