What you think you're doing: "Just following up" (and showing off how organized you are in the process).
Why it doesn't work: It's easy to assume the only reason you haven't heard from someone is that the person's a jerk; but, more likely, she's swamped, waiting on budget approval or putting HR paperwork through. Asking for an answer with bullyish language, such as, "Please confirm that you received this," is a great way to ensure you'll push anyone who's on the fence about you in the wrong direction.
What to do instead: After you've sent the thank you note, it's okay to check in once, or even two times; but, after that, move on. Otherwise, says Slayter, "You're in stalking territory."
Next: 6 reasons you are making less than you should