Practice, practice, practice. This is the only way to go. I bought a whiteboard from Office Depot, googled some design challenge topics and started scrambling away. When I was whiteboarding, I timed myself so that I kept the entire process around 30 to 40 minutes. I talked to myself loud as I progressed — very important to interact with the interviewer during whiteboarding — it was fun, haha :)
… or HR. It lasts from 10 minutes to 30 minutes. They almost certainly ask you about visa status and desired salary (good to give a range rather than a number). Some would ask you about your professional knowledge, for example, what your definition for user experience is, how you carry out user research.