What NOT to do

Skip the Back-of-the-Envelope Math

This is when at the beginning of the interview you start asking: how many users do we have, how many requests per user do we expect, and so on.

5 minutes asking questions. Then 5 minutes showing off that you know math: "Okay, 1 million users, 3 requests per user, so we need this many petabytes of storage and the network throughput has to be really fast." That's 10 minutes gone.

Don't waste time here, because back-of-the-envelope estimation almost always ends with the same conclusion: "Hmm, this is a big system."