Asking in person can be effective because people are less inclined to say no to you even if their inclination is to say no. People dislike saying no to a live person. With letters sayin no is easier. It is also easier to passive-aggressively forgo a response. Many people who would be inclined to say yes don't seem to have the time to respond, so they tend to put your request at the bottom of their list of priorities; however, a BRIEF well written letter can inform them of your intentions at a time that is convenient for them. Would be awkward to to ask if something bad had just happened in their family. Then if you don't get a response, call them up. Now you don't have to take the time to fully introduce your intentions, which makes the call that much easier for the both of you.
Make contacting you as easy as possible. If you want a written response, include a self-addressed, stamped envelope and make sure the envelope is self-adhesive- you don't want them to remember as the guy/gal who sent a SASE that, when licked to seal the envelope, it tasted like feet. When including the phone number, specify if it is a cell phone; many people prefer texting (texting is super convenient and an awesome tool when used properly). Include your email address, but make it an easy one. Pk2871buz@xxxxxxx.net would make it that much more difficult for them.
Property owners hold their property rights in high esteem, as they should, and dislike when others trespass. The owners often appreciate the heck out of someone who respects their property by asking permission. You will undoubtedly run into some crotchety old bugger that lives his/her life for the sole purpose of jerking other people around; after absorbing some of their negativity and even hostility, move on. Most people are reasonable and nice.
Finally, sometimes it is better to ask for forgiveness later than to ask for permission first. This is not a recommendation to knowingly commit a crime or an unethical deed. In some cases like on public land, the government employees will say no in a knee-jerk response no matter what the law is or isn't. This paragraph is a bit off topic as you are asking about private land access, but it is good to keep in mind.