Glenn,
It is a good idea to start asking around now. Sometimes it takes a while to get permission and as posted above, if you ask now and they say no you still have a shot at 'working' on them. Never give up a good lead. I have been going through a realtor working on a site for well over a month and will wait it out.
It also helps if you can approach the owner in person and start a conversation based on something other than metal detecting then tell them how much you admire their property and maybe get them to talking about it. Give them a chance to get to know you, even if for just a few minutes so they can size you up a little, then lead the conversation around to searching their property. This is much better than just walking up and telling them you want to metal detect their property out of the blue, but knocking on doors and ringing bells to get permission on the spot does work more often than not.
In your winter research try to find as many potential hunting sites as you can and make a list of all of them. Take a day to drive around to all the sites and see if they are still open or have been paved over and built on. Out of a list of 20 you may only find 2 or 3 that are still open. Keep notes. Look for realtor signs or do your time in the county court house in search of owners if you need to.
Don't be too disappointed or feel rejected if you get no for an answer. Just have enough sites on your list so if some of them don't develop you can still score on some of the others.
The biggest objection you will run into is the liability factor. You can counter that by offering a liability waiver. If they still say no then that isn't the real reason. They just don't want to bother with it. Also, have an agreement form in case you have to negotiate a split of the finds. Of course you always want to start out without offering a waiver or an agreement form. Only bring it up if they do.
Always be sure you are dealing with the owner, not renters or relatives. If the owner says yes and it's very casual then go for it. If the owner even mentions a split then be sure to have a signed agreement with the terms laid out ahead of time and have two copies, one for him and one for yourself with both signatures. It would be a real shame to run across the mother load and not have an agreement in place.
Here is a link to a thread from the past that includes information about finding leads. It is a long read but there may be some ideas that will help with your winter research and finding sites to hunt:
http://forum.treasurenet.com/index.php/topic,120170.msg882262.html#msg882262
I have a form I have used in the past that I can send you if you would like to use it or get some ideas from it. Just pm me. HH and good luck. Highwater