While the best way to fall is not at all, falling is an inevitability, and while crash pads reduce the impact considerably, injury is still a possibility. The best way to fall is to flex your legs as you land, distributing the force of the fall, and eventually to sit. So, in other words, first your legs, and then your butt. Try to avoid falling forward, although this is not always possible. Always try to throw your weight behind. If you are unsure of the best falling technique, try asking the staff at your local bouldering gym who will, more than likely, be happy to show you the safest way to dismount the wall.