I think going door to door is a waste of time and energy. You will look completely inexperienced. Not only that most people are in denial that they are in foreclosure, dont want to talk to anyone about it, depressed, pissed off, etc... I wouldnt consider it! Learned the hard way and then the easy way by getting educated.
Send letters that indicate nothing of the fact that you are here to buy there home, get them out of foreclosure, work with the bank, etc...
Send them a letter saying that it is unfortunate that they are going into foreclosure and if they need any help to contact you. Dont mention the down falls of foreclosure or any negative writing to them, they already know that and dont care to hear it.
Wait about a month and send another letter saying the same thing, but a little more in detail and letting them know you specialize in helping individuals potentially get of their foreclosure. Usually by the second or third letter they will call you back, because you come across as you care and not there to hound them and promise to buy their house or get them out of foreclosure.
Hope this helps!