[truth_be_told]

In order to be aggressive, you should train yourself to not be afraid of any possible consequences and sacrifices. Thats it!.

Most of the time, our imagined dragons/troubles never come to pass. People imagine all sorts of dire consequences and therefore do not act aggressively when required. You need to learn to push boundaries, keep your stress under control and always remember that the "other party" is as afraid of consequences as you are. The trick is to keep the pressure on until the other side "blinks". Watch any Nature conflict documentaries (eg. Lion documentaries where pride takeover often happens) and the principle should be clear.

Brinkmanship is the name of the game.

A study of Martial Arts texts will help in building up the necessary principles/techniques and "mental fortitude". A Book of Five Rings by Miyamoto Musashi is a good starting point.