The only way to really put pressure on your opponent is to break the advantage line. Every turn you must make sure that your team are moving the ball forward and past it's last position! Try to get behind your opponents defense and set up a Ruck.
This forces them to turn and fall back. Remember players ahead of the ruck are considered off-side and are in effect useless to defend, so the more you force your team behind the defenders the more pressure you put on your opponents defense!