This is a very interesting subject! I moved into this neighborhood 10 years ago and most the residence are older. People quickly came to realize that I was the most capable person on our road. I removed many trees, a lot of snow, cleaned gutters and I even pulled out a neighbors car that went over a bank in a snow storm at 1AM on a week night after I had already gone to bed.
At first I was happy to do these things and would didn't accept tips. After a few years, people expected it but it was getting harder and harder on me as the requests piled up (there is only so much one person can do). One year I hit my tipping point. I had spent 6 hours clearing driveways with a 19hp open station manual tranny in a very wet 3ft snow storm. I was on my forth and last pair of gloves and they were soaked through. My core was cold and I went inside at around 11PM and sat down shivering next to the stove as my wife went to get me a warm drink. Then the phone rings. A friend who had just spent the whole day skiing was calling because he couldn't get home and wanted me to pull his car out of a bank and over the last hill so he could make it home (he was within walking distance of home already). He was a good friend so I helped him out. But really began to question why I felt compelled to respond to every request!
The next day a neighbor calls and is kind of rude to me wondering why I didn't clear his driveway. I went on a rant and told him it's not my problem and that he will have to pay a contractor the going rate of $150 to clear it. From that point on, most the people in the neighborhood starting giving me nice tips when I helped them out (I guess they talk). I accept the tips and I also say no when I don't have the time and I am much happier for it. I get enough tips each year to buy a new implement which everyone in the neighborhood benefits from and I feel less like I'm being "used".
I still do way more for my neighbors than they do for me and I fully expect it will always be that way. The tips are just enough to cover fuel and wear / hours on the tractor. But I don't stress about not helping. If I'm busy I say no. And when there is a big storm, those who tip best get the first service (well, right after the widows) - I figure they must be the ones who appreciate it the most. The ones that don't tip - I may or may not get to them.
The point is, don't let it get out of hand. If it's just a few requests then no big deal. If you get to the point where these requests are stressing you out, squash it before you flip out on someone. In my case, flipping out on someone didn't hurt any relationships but it could have. They apologized and went out of their way to patch things up. After-all, I'm still the only guy in the neighborhood that people can call when they need help. And people seem much more appreciative when I do help them.