Raising a tough kid means raising them in an atmosphere of safe risk.
Children are mostly instinctual so it is not that hard to observe the right times to push the kid and the right time to soothe. If the child panics over a task you back off a bit, wait for them to calm down, and logically explain that the task is not as scary as it seems. If the child is not in a state of panic but being a wimp, feel free to challenge it and push them a little.
Another thing is to place broad boundaries of respect that allow for movement with out over dominating them on details. For instance areas where they can play with or do anything they like, but if you are in the dining room you do not touch. Letting them explore on their own. Letting them complete tasks on their own.
Today in the pizza shop I told my three year old to wait his turn in line then ask the girl at the counter for a to go box. It took him five times returning to the table before he got it down, but he was the proudest kid when he actually got the ladies attention and came back with one. The whole time we were there he was pressing to go into the small arcade portion, and I told him to wait. Then right as we were walking up to it, a field trip buss of kids swarmed the area and took up every machine. But I just gave my son 50c and said okay have fun. I positioned myself in an area where no one could come in or out without passing by me, knowing the area was secure for him to explore on his own. He got lost in all the big kids for a second then emerged and somehow took over the one game he had been wanting to play. After that I joined him on the game.
I guess making an atmosphere that is enforced by rules and boundaries, but encouraging the child to explore on their own. If they feel safe they will be confident, and you can guide it from there.
I guess the best way to create strong kids is to create the illusion that they have choice, freedom, and ability. Most of my parenting goes on behind the scenes. Creating the world he moves through, but allowing him to move at will.