The right posture not only helps avoid a sore back, it also means better control of the raft by the team as a whole ― especially important when you start plunging through rapids.
1 Sit. Perch on one of the raft's side tubes; tuck your feet into the raft's straps or floor pockets, if available, or under the thwarts (the tubes that cross the raft).
2 Grip the paddle. Put the hand that's toward the center of the raft securely over the T-grip at the top of the paddle, and keep it there at all times (so you don't thwack your raftmates). Put the other hand near the blade.
3 Stroke. Coordinate with the person in front of you to avoid knocking paddles. When forward paddling, slice your paddle into the water in front of you and pull it toward your hip like you're trying to retrieve a leaf on the surface. Back paddling is actually a stronger stroke because of the way your back and leg muscles are used for leverage.