I'm no expert on ponds, but have had one for about 7 years. Where I live we have problems with herons. Therefore, I have to cover my pond with fruit tree netting to keep them out...the added bonus is it also keeps leaves out. I just get the leaves off the top once a week or so as needed to keep from piling up.
I have a power box wired from garage nearby to my pond. I use a double filtration system that I designed and that keeps majority of smaller particles out of the pond. My water is generally clear, less a little bit of algea on the sides. Some algea is good, my fish love to eat it. Some would argue you need to feed them, but I don't feed regularly with all that natural food.
As far as freezing, the climate here never has frozen more than a thin layer on some of the pond. But in the case you do get a solid sheet of ice, melt it with warm water, don't break it, to allow the toxic gasses from decayed leaves etc. escape. The heat and water movement produced by the pump in my pond helps too. I also got some cheap tubed outdoor lights from Wal-mart and siliconed the end closed to keep the water out, that serves as cheap lighting and heat. I never remove my fish in the winter, they just go to the bottom and stay dormant until warmer weather.
I usually do about three big cleanings a year 1) Spring: to get rid of any die off of plants in and around the pond from the winter, 2) Summer: to cut back on the overgrowth from some of my fast growing plants, 3) Fall: usually just before it gets cold, I get rid of the plant material already showing signs of die off. At each of these times I also use a silt net to get any left behind sludge out of the bottom.
It is a ~250 gallon pond. In the pond are a bullfog, goldfish, koi, shubunkins, lots of plants. Not sure what is left after a baby garter decided to grab some dinner a couple weeks ago. I don't like snakes and don't wanna stick my hand in there.