Different parts of the country (US) get different results. I live in the midwest and have, I'd guess, about a 90% success rate. Don't knock on holidays when family visits are expected. Dress for digging, but be respectable. Don't smoke or chew. Don't carry your machine and tools to the door. Approach by yourself, not with a hunting partner. Knock, wait with your back to the door until you hear someone coming to the door...do not look in windows. Put a smile on your face and be clear, concise and polite. Tell the person(s) your name (and your buddy's name if he/she is waiting in the car), where you're from, where you work (or go to school) and that you're out metal detecting, which is a hobby of yours whenever you can find time from chores, work or school to get out....use the word hobby every single time. That's it, expect a yes, or a no, or a let me ask my husband/wife or some questions. Be prepared for questions. Do you dig? Yes, but I have a professional tool that cuts a very small plug (enact with your hands), I pull out the circular plug, pull out whatever was in the hole, replace the plug, and you can't even see it. Carry some of your dirty coins with you so you can show them what kinds of things you find...some people always ask that question. What if you find anything good/valuable? Ma'am, Sir, this is your property and I'm out here because I love finding history. I will show you everything I find, and what you don't want, I'll keep. Do you mind splitting 50/50? That works for me, I'll show you everything I find. If you get the go ahead, be respectful of the property, pets and people, because word-of-mouth travels fast, and that can either help or hurt you. Dress up your holes as you go, don't wait to do it at the end...use a towel, plastic, frisbee, etc to put your dirt on so you can actually be proud of not leaving a trace. Use your success at one house to go to the neighbor's house--use the same approach, but start with, "I was just at Mr & Mrs Jone's house next door and..." It always helps to have an icebreaker.
If you get a "no," just say ok, but before I leave, can I ask you if you know what the history of this neighborhood is? Do you happen to know what year the houses on this block were built? These look about like turn of the century homes, is that about right? Hopefully you'll get a knowledgeable response, which sometimes can lead to a "no" becoming a "yes." Each situation is different, and being able to read tone and body language is a key talent that'll come with experience.
There are hundreds of suggestions, but you get the picture...good luck!