Hi Mike,
It depends on what your state's foreclosure process is. I'm in Illinois, which utilizes the judicial process. (hope I termed that correctly!) 8)
Anyway I'll try to answer your question in three parts.
Early stage: The property still belongs to the owner. Assuming you have
contact with him/her, have them sign an "Authorization to Release Information" form. The lender won't talk to you about the deal unless they
have permission from the owner. Then you can begin negotiating. You'll want to ask for the Loss Mitigation Dept. Sometimes it'll help if you just ask the lender what needs to be done to make the deal work.
Already foreclosed: Now the property is an REO. However here in Illinois
there is a time period of about 30 to 45 days, between the actual sale date and the court approval of the sale, where you still might be able to negotiate with the lender before it gets listed. Of course once it's listed then you have to deal with the listing agent.
Empty for months: If the property's been vacant for a while, that doesn't
necessarily mean that it's an Reo. Have a Realtor friend look it up to see if it's listed. If not, then leave a note at the property in question that you have an interest in buying it. Talk to the neighbors to see if they know the owner's whereabouts. Leave them with your contact info.
In summary it takes a lot of patience dealing with the lenders and some savvy detective work finding the owners. If your budget allows I would subscribe to a reputable foreclosure service.
Hope this helps and GOOD LUCK!!!