Simply by telling them, they are adding a barrier to their goal of selling their home.  A

Jillian - Like most of the other answers, you just need to let the homeowners know that any potential buyer will feel uncomfortable going through a home with the sellers there. They certainly won't stay as long, and may not ask any questions. If they do ask questions, the seller may reveal something that can be advantageous to the buyer if they do decide to make an offer.

Reassure the sellers that anyone looking at their home will be accompanied by an agent.

Jillian La Rocque I just tell sellers, the buyers would like to view this home as theirs and want no intruders! Give the buyers their space! It works most of the time.

I tell them the truth that IN GENERAL it makes people uncomfortable.

Jillian,
I explain that it is crucial that the potential buyers visualize the property as being theirs. The seller being there kills that visualization.

Create it as an expectation when you are discussing the listing and signing the paperwork.   

I tell them a horror story or two about what happened when sellers stayed home, including the one where I came with buyers who refused to get out of the car and look at homes when anyone was at home. 

 You have to remind them that buyers can't try on the house with them there - welcome to Active Rain !! 

I tell them why iit is so important and often add a story about the past deals gone wrong, if they say they would like to stay.

I explain how it is not in their best interests since they have very specific opinions about the property that my be difficult to contain when a tranger brings up a negative. Most understand and the others are free to list with someone else!

Discuss as part of the "rules of the road" before signing the listing agreement...

I mention at the listing appointment.

I am open and honest with my clients.  I really don't see a problem with showings unless it's a tenant that doesn't want to move.

I just tell them the procedure is that they not be present while their Home is being shown because it makes the buyers uncomfortable.  I explain that the showing Agent will accompany any buyers and random people will not be walking through their home. If they insist on being present I tell them to refer any questions to me and not to interact with the showing agent other than to say hello and good by.

 Good morning Jillian. I don't know what the one can be tactful in that regard. I know that they are curious, but they need to understand the rules of the game.

Just have to tell that this is the way the game is played, and if they want to sell they would be wise to make it more comfortable for the buyers to snoop around and discuss the home before writing up an offer. 

I tell them Buyer are very uncomfortable when a Seller is present and it hurts their chances of getting interest or an offer.

I just tell them they need to leave.

You just outright tell them that the buyers will need to discuss the home privately while they are viewing it and it is best if you are not present.  It will make them nervous and all they will think of is getting back out of the home.

I discuss this at the listing when the issue has not come up yet and say buyers are uncomfortable looking when a seller is home because they feel like they are intruding. If a buyer can't look at everything they are likely to not buy so I get agreement from the seller they will leave. They want the buyer to buy.

I just let them know that to get is sold most buyers want to see it without interuptions or be feel like they are being watched. In our market is is quite common though and almost everyone is ok with it!

Hehehe! It requires a gag, handcuffs and a bullwhip.
Seriously  . . . "Mr Seller, I would prefer you disappear or make yourself very scarce during showings - this way the buyer can begin to 'bond' with the home. Buyers won't buy what they can't envision themselves owning."

I've held open houses where up front at the listing presentation the seller said that they will be present for every open house and every showing --the house is still sitting on the market.  It definitely turns buyers off when the seller is there.

I appeal to their desire to sell the house and explain to them how this turns a buyer away from the house and really looking at it. 

Personally, I don't think it's a bad thing to have the property owner around, they should just not interfere with the showing. You might also tell them that buyers feel strange being in their home with them around; they won't look at it as much as when only their agent is with them. If they trust the listing agent, maybe he/she should be there unobtrusively instead of the owner. I've had listing agents there who could be very helpful in pointing out things about the property.

I give sellers their insturctions in writing and tell them there will be a test.  They know I give the same information to everyone and don't take it seriously.

That is a hard one, but I always tell them if I was the buyer agent I would figure out why you are selling and reduce the offer onces I knew the motivation

I would remind them of what would they want if they were looking to purchase a home.  Would they want the seller present while they toured the home and made comments or would they want the sellers absent during that time?

That buyers want to take their time and talk freely while in the home . With the Seller still there , neither will happen.

In most cases, people will understand when it is explained.  There are some situations, where it is not possible for the sellers to easily leave, so I explain what they should do in such a situation.

The easiest way is to say I woll not list the home if you are present at showings. 

I agree with Ron and Alexandra Seigel and if they still insist I let them know that a criticism actually shows their interest in the place and in no way should they respond negatively. Sticks and stones may break my bones....

It really depends on the case, the people. But really, your Clients should trust you as the Agent to just simply show the home with no problems.

It's case by case. A large % of sellers will voluntarily leave for open house. They have heard and read many times over the years that that is usually the best procedure for making potential buyers more comfortable while viewing their home. No one wants someone hanging over their shoulder while shopping. Other sellers think they need to stalk and park across the street in a neighbors driveway, but at least they aren't in the house. Those type are still annoying, and usually easy to spot. Some work from home, are elderly or handicapped, or have legitimate reasons for not being able to leave. Most will stay out of the way, and only talk when someone asks them a question. Only a small % insist on being present, think they have to be a tour guide, and blab about personal preferences that most buyers have zero interest in. They only hurt themselves by distracting the attention of potential buyers away from the property.

I'm not weird about people being home during showings and I'm offended when another agent makes me and my client wait outside while they show the home.  There are many homes to see and the fact that people allow us into their home is appreciated.  It would be great if the home were empty but in many cases sellers have sold their own home by sharing with potential buyers.

Understanding the seller hope is to sell their home for the highest price as fast as possible. If your seller has different objectives you really need to find out what their objectives are.

Now I would role play, being the prospective buyer. Playing the innocent and friendly buyer, direct a few charming and innocent question towards the seller. "This is such a lovely house you have. Why are you moving?"

Seller: "I love living here, just too much traffic. Take 35 minutes to get to the drug store two miles away."

Buyer: "I see, In the years you have been here what have you come to know about your neighbors?"

Seller: "This is very safe area. Anyone prowling around here will set the neighbors dog off and that will send the prowlers running. And Ms Jamison, in the blue house, she keeps an eye on everything. Occasionally here son comes back on release and things can get exciting but blows over soon enough."

Now show the seller how much engaging in polite conversation will cost.

"You will get more money is you stay away during the showing. If that is not important to you, feel free to share all you know with the buyer."

In your conversation with the seller point out each point that will alert the buyer to a deficiency.....unexpectedly long commute time, traffic aggravation, barking dog, intrusive neighbor, unruly behavior.  The property has already been devalued and we haven't even got to the house yet.