Make it Clear: Search is for Search and Email is for Email
Often times retail sites offer search and email signup boxes that look almost identical. It's very easy for customers to mistake one for the other (you'll know if your email signup logs contain "addresses" like "nose hair trimmer.")
Rather than leave them blank, the retailers above label the fields with instructions like "keyword or item number" and "your email address."
It's simple, but it can help prevent many errors!