Avoid options. If there is any possibility of confusion about the scope of the search (what’s being searched: the site or the whole Web; by all means, spell it out. But think very carefully before giving options to limit the scope and providing options for how I specify what I’m searching for (author, or product name).
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.