I can't go into specifics, but I thought the first part of the interview wasn't too hard. Tip for coding: go for the simplest solution. As for the skill assessment, GRE quantitative reasoning tests will help, but I thought that enough time was given that you don't really need to prepare for a speed test. I would recommend just writing whatever programs you would normally for fun for the in-person interview. As long as you're comfortable in one of the common languages, it's not too bad.