No matter what you delete, you're still the same UA (or finite number of UAs) from the same IP or known-to-google IP range.* Do your searches from a genuinely public terminal such as a school or library. For some searches, this itself will be skewed (results targeted to people with a given educational level, or to lower-income people who don't have their own computers) but it's probably as close as you're going to get. Try your laptop from an Internet cafe and the WiFi area of your public library, and split the difference.
* You don't meet a whole lot of professional webmasters on dialup.