You just have to get good at it...that's about what it amounts to.
For hills...you need to work on the transition of your right foot from the brake to the gas in a hurry to make sure that when you get that clutch out you have enough gas so that you don't kill the engine.
As far as slowing down....if you know you are going to stop, you can do whatever you like. However, when you are approaching a light and you might not completely stop, it is wise to downshift. Not only does the engine braking help slow you down, but you won't have to all of a sudden think "Damn, what gear now" once that light turns green.
Honestly...the STi is the easiest manual car I have ever driven because of the low end torque. You shouldn't have too much trouble with it.