Mark,
I would not beat around the bush about why you are there. I would learn about the options someone in that situation has, and how each option impacts them. If they have gotten that far downstream they are probably staring down the foreclosure cliff. I would develop the netwrok to help them cash buyers, other investors that would be interested in a Sub2 and the like.
I would tell them why you are there, "... you are headed to foreclosure I work with homeowners in your situation....are you interested in selling your home?". Be courteous, informed, and you need to be truly interested in helping them with their situation. That way you don't waste yours or there time, or muck up a realtor's name.
It's not an accident your at their doorstep. You are there to solve a problem and conduct a transaction. I would present myself as very capable of accomplishing this.
When are you doing this? Afternoons and weekends will net you the most contact. I particularly find weekday afternoons to be the best.
Do you know you if you are targeting the best prospects? Make sure to spend this time, because it is time and fuel consuming, on prospects with at least 30% equity in their home. That is my two cents, and what has worked for me.