What can I do to either prevent or soothe aches in my knees after a day of skiing?

Using proper warm up and stretching prior to skiing and more importantly by installing a proper condition program prior to the season best accomplishes prevention of knee aches.  For help with aches and pain after a day of skiing I recommend proper cool down; quadriceps, hamstrings and Illiotibial band stretches; ice; massage; and prudent use of Motrin or Aleve.  At a minimum find some place for stretches after skiing and prior to that car ride home. These pay dividends for helping knee aches.
Some athletes have found that Kinesio taping (KT taping) helps with various activities, and is very popular among some high profile athletes.  Individuals need to be aware of swelling in the knee or effusion, as this can often be a result of cartilage injury.  If recurrent swelling is present with activities, I recommend you come see me.