I haven't really had much experience at asking myself but I've read quite a few comments from others on this over the years. Obviously you should never show up at the door in camo, the day before season. I'm not saying you should show up wearing a suit and a tie but most recommend wearing nice clothing and looking respectable, no hats or whitetail this or that clothing. I would recommend making up a resume so to speak that has some personal information about you and maybe even references. That would be something you could hopefully leave behind in an effort to get them to think about it and perhaps when you return a couple weeks later you could ask if they would reconsider (provided you were turned down in the first place). Remember, everyone who walks up to their doors is suspect and they are reluctant to welcome just any stranger without first trying to get to know you, that resume could be the one thing that changes their mind. Information about yourself could include everything from where you live, school information (college & studies?), how long you have been hunting, anything that shows you are an upstanding person, etc.. Might be a good time to tell them what hunting means to you and it's importance in your life, hopefully not just killing the biggest buck they have on the property. You can also list things you would be willing to help out with on their farm.