Well it depends on how your team uses its flankers. If you're the blindside your primary responsibility in the scrum is to stop the opposing 8 from sneaking around the weak side. If your team just plays right and left flanker then your responsibility off the scrum is to put pressure on the opposing teams flyhalf, if you're on the openside and be ready for the switch with their number 12. In line outs you're either going to be a jumper or a lifter, defensively you should be ready to burst through their line and put pressure on the opposing scrum half. I personally don't subscribe to the belief that the Flankers need to be big hitters, their main job is to maintain possession for their team i.e. be at the rucks to prevent the other team from poaching and to steal possession from the other team, either at the breakdown or in open play. Offensively a flanker is the link between the bigger slower forwards and the speedier back line. Either taking pops off bigger forwards who can use their size to open holes or drawing contact yourself to open holes for back line players. Assuming you've got decent hands (and it's important that you be honest with yourself here) you can also insert into the back line and provide support for moves like switches, skips etc.
offense: get to the ruck. you dont have to be at every single one, so dont chase the ball, because youll wear yourself out. if you cant get to one, put yourself in position to get to the next one. the best way a flanker can help his team is to get to the rucks.
