This drives me bananas as well. There are a few things that can be done, but it mainly comes down to giving up conveniences that we’ve come to think of as necessary. I’ve done everything I know how to, except the only thing that I think will work, which is to always, without fail, log out of all things Google before going to YouTube, so that I look like a new user without history.
What I have done is to turn off my YouTube history. I think this is having some kind of effect. I don’t get a record of the videos I have watched. I do get recommendations based on what I watched, and I do get lists of what my Google+ friends have shared. Here’s how to turn off your YouTube history:
YouTube > Arrow next to your avatar at the top right corner > YouTube settings > Video Manager (from bar across the top) > History
I think there are options for you to have YouTube stop collecting “history” information on what you watch and (note the separate tab for this) on what you search. Infuriatingly, Google has started personalizing options depending on prior settings, so I can only see how to resume my history, not how to stop my history.
Likewise, your Google account has similar settings to tell them to stop collecting data on what you have searched in the past, and I have those selected. Because of the way they have synched their products, I suspect this may have some small effect on my YouTube account also.
But finally, even though I have done these things, I still get videos “recommended for you” based on what I have viewed. Logging out makes these disappear. It used to be that clearing history would eliminate anything for YouTube to base suggestions on, but that is clearly no longer the case. We appear to be stuck with reminders of what we’ve viewed, no matter what we do, as long as we are logged into YouTube, Gmail, or other Google products.