How do I get the swelling down in my knee after sports injury?

Ice. The fastest way is ice, elevation, compression and anti-inflammatories. If you get swelling after an injury, this should be evaluated as it could represent a fracture, ligament spain or meniscus tear.
Ice. Apply ice to the affected area, 4-5 times per day, for 20 minutes at a time. Take an over the counter anti-inflammatory medication.
Ice after sports. The best way to minimized swelling if ice after any workouts or sporting activities. Do not use heat. Ice will decrease the swelling and along with nsaids can help resolve your symptoms in a rapid manner.