You've got to be a salesperson. Sell yourself. Look in the mirror. Do you smile or frown? Is your smile genuine? People can see right through a fake smile.
Are you clean? Scraggly? Unshaven? Clean hair? Knocking on the door of an old house most-likely will produce old people at the door. They take note of these things.
Are your clothes clean? Are your clothes conservative? Its another thing people look at. If you are wearing grungy pants hanging off your butt, I'll bet you will find that is a detriment to successful permission-asking.
Speak evenly and with assertion. Know what you are saying, and say what you mean. Dont cloud the conversation with gobbledygook that the person doesnt understand. The ABC's of good communication are ACCURACY, BREVITY, and CLARITY.
1. Tell them exactly what you'd like to do and where you want to search.
2. Be brief. Don't ramble on. Dont waste their time or yours.
3. Be specific. Be clear. Be simple.
Do all this above and be sincere, and you'll have no problems getting some permissions. What I do, after I hunt is tell them I am leaving and thank them profusely for their permission. I also try to fit in a, "and can I please get your name and address so that I can put this in my log book?" If they consent, I send them a thank you card and a Christmas card. Its an added expense, but you would not believe how many extra "add-on" places that I have gotten to hunt because the first people told their friends how nice I was. You catch more flies with sugar than you do with vinegar!