The best openside flankers offer a ball carrying option when attacking, and are an ever present thorn in the side of the opposition attack. As discussed above, make sure you are quick and fit to allow you to always be one of the first to the breakdown, and just learn to make a nuisance of yourself when you get there - legally slowing down what would have been fast ball for the opposition is one of the greatest services you can do for your defensive line.
You don't need to tackle hard (although it helps), but you should always get your man down, and if possible be back on your feet to contest the ball (this is where the fitness really pays off as the game wears on). Strength training obviously pays off, but the real focus is on fitness, speed, and technical ability - learning exactly how to slow down and contest the ball legally.
Reading of the game, as also mentioned above, is crucial. Knowing where your backs intend to attack will let you be there to make sure they get the ball back rapidly in the contact area, and prevent easy turnovers. You also need to position yourself correctly on defence to make sure you can drift across the line to cover tackles and get to the contact area, but not drift so quickly that you expose your inside shoulder to switches etc.
A good openside can really make a team, so good luck with it!
