There is no magic bullet for this - its all about confidence and also knowing a fact that we are not building a rocket :-) . Experience helps (to be precise a lot). Working/getting mentorship with another good archiect would also help. In my opinion, it is easier to learn the tricks of the trade while working with consulting firms as they normlly pair archietcts usually an experienced one with a junior when sending to big projects. Learning additional items on your own wouldnt hurt but they all matters only with a working experience.
The more the varied IT experience you have, the better you will be in this role based on my personal experience. But I have also seen people with less hands on experience in technology but worked as architects from the begining and those people are invariably came from the Big 5 Consulting firms (Like accentue, E&Y etc)