Have shelves and drawers ready for all your teenager's belongings.
Teach your teenager how to keep his/her room tidy. Encourage him/her to vacuum, sweep, fold his/her clothes, etc.
Help him/her clean the first time around.
Draw a floor map of your teenager's room to make it easier for him/her to remember where items belong.
Reward your teenager for keeping his/her room clean.
Insist that your teenager gets rid of the things he/she no longer uses.