This may seem obvious. Putting yourself in a position in where you have to regularly, if not constantly, be assertive will help tremendously.

I believe assertiveness can be roughly defined as: demanding attention or action with little to no preparation. Sure, almost anybody can approach another person and make a request with time and preparation - but getting others attention or demanding action on the spot...most people, including myself, become hesitant and/or falter.

Before my current job, I was just like that. In my current profession, I'm largely responsible for safely transporting thousands of passengers every day on a large marine vessel. Most of these people are transporting by automobile and have to safely navigate them onto the vessel and park in a safe/efficient manner. When these auto mobiles (cars, trucks, semi-trailers, busses) roll on to the vessel, it's literally a constant stream in which I have to make on the spot assessment of space, entrance/exit accessibility depending on size, and access to elevators for handicap vehicles. I do all of this while directing them where and how to park. Quite frequently, I will have to stop the flow of traffic and get a passenger to repark their car as they are blocking a dozen + spaces.

We're also responsible for any emergencies including fire fighting, man overboard and evacuations.

It's safe to say the first couple weeks were tough. I struggled quite a bit. Even the slightest hesitation could ruin a good portion of a vehicle load. It literally forced me to become assertive to remain employed.

I even noticed improvements in my day to day life. Approaching strangers or service people in public with requests can be done with complete confidence. The main difference is I don't over think the consequences. That's not to say I don't know they exist - I just feel I can handle those consequences if they do arise.

Seriously. Volunteer, join a team sport, or find a job that forces you or at least encourages you to be an assertive person.

Good luck man.