Have you ever heard that toddlers and children are the best negotiators and they do not even know they are doing it? You maybe negotiating without realizing it too.
Here are the ways you may be losing your place at the table:
Calling the listing agent to show you the property because your agent is "busy" or "out of town"
Asking the listing agent if they will represent you as well as the seller, to get a better deal
Going to the open house and making negative comments about the home, I rarely hold open a home that I do not hear "well it needs a lot of work"
You are pre-negotiating; if you call me to show the home, know that I will call your agent to see what the story is, often the buyer's agent has a stand-in for when they are gone and they can show it. I do not mind showing it. But because side stepping your agent, you as the buyer may come off as a bit shady. Keep in mind when this is done it is hard to come back from when presenting an offer.
You are pre-negotiating: if you are looking for a better deal asking the listing agent to represent you this is a strategy and sometimes it works. It will not work for me as I only work for one side that guarantees the get the absolute best possible price. Also I have talked with many people and in the end they were never sure they got the best price. And again if you request that of a listing agent it is pre-negotiating and may not work out in an offer situation later.
You are pre-negotiating: if you make disparaging comments about the home. Put on your game face no negative comments about the home, this is telling the listing agent you see less value in the home. I guess the idea is to make the agent question the value of the home themselves? But when I hear "this home needs a lot of work" I think the buyer does not understand the value of the home or "I want to offer less and I am testing the water" One time I had clients that would view homes with me and they were always very complimentary to the listing agent and said great things. But when we got in the car they would say " this home is not for us" such a perfect strategy. They kept all opinions to their selves and then thought about it.
You are of course allowed to do what you feel is best is for you. This is based in experience and over all I recommend keeping your cards close and communicating with your agent directly as the way to get the best outcome
ps you have to be very very careful what you say in an open house or a showing the seller could be lurking around or using a listening device. Any negative comments could give up your position and pre-negotiate you out of consideration.