What can I do to reduce swelling and scar formation after a major accident?

Elevation. There is not enough information to answer your question. As a general rule elevation will reduce swelling, scarring is the healing of a wound and is necessary, more information will need to come from your treating physian[s].
Pressure. Pressure will reduce swelling and scar formation. The mechanism for this improvement is a sunject of debate. If the injury is a limb then gentle wrapping with an ace wrap from the hand or foot up to the injury will help. The wrap should be snug but not painful. If the scar is not on an extremity then tape and a silicne pad may help.