How to keep pumpkin fresh for a long time?

According to the National Center for Home Food Preservation (emphasis added):

Freezing is the easiest way to preserve pumpkin, and it yields the best quality product. Select full-colored mature pumpkin with fine texture (not stringy or dry). Wash, cut into cooking-size sections and remove seeds. Cook until soft in boiling water, in steam, in a pressure cooker, or in an oven. Remove pulp from rind and mash. To cool, place pan containing pumpkin in cold water and stir occasionally (So Easy to Preserve, 2006). Pack into rigid containers leaving headspace, and freeze.

They offer suggest a variety of cooking methods before freezing the flesh of the pumpkin, but you may find that roasting offers the least work and best flavor development.

Pressure canning of cubed (but not mashed or pureed) pumpkin is also possible (as pumpkin is a low acid fruit, boiling water canning is not appropriate). You should use only a trusted recipe if you choose to go this route.

You can also make pumpkin leather, although this does not to leave the pumpkin suitable for many recipes, and is only suitable if this the actual product you desire.

I clean out and cut fresh pie pumpkins into about 12 pieces each. I use my pressure cooker to cook the pumpkin pieces...about 15 minutes under pressure. Once the cool, the skins peel right off. I then mash the pumpkin into a purée.

I put 2 cups each of purée in 1-quart freezer bags...about the amount in a typical US can of pumpkin. Then freeze, of course.

We just used some year-old pumpkin in a batch of pumpkin bread, and it tastes as fresh as when I made it. Great for pumpkin pie, bars and pancakes. Nothing better than using homemade pumpkin purée with fresh spices!

I have had a great deal of success with turning the pumpkin in "Pumpkin Puree" and freezing it. The puree can be easily portioned and stored in freezer bags. Once thawed the puree can be used for any purpose.

Some pumpkins and other hard squashes will survive 3 months storage as they are, as long as you leave them intact. Optimum storage conditions are described on legions of gardening websites. This http://extension.oregonstate.edu/lane/sites/default/files/documents/ec1632.pdf extension office document suggests to "Cure squash and pumpkin for 10 days at temperatures of 80 to 85°F and a relative humidity of 80 to 85 degrees." (that has probably already happened if you buy a pumpkin), "For best results, store sound, well-cured fruit at 50 to 55°F in a 50 to 70% relative humidity." , "Squash and pumpkin deteriorate rapidly if stored at temperatures below 50°F", "Keep the surface of the fruit dry", "Do not store pumpkin or squash near apples, pears, or other ripening fruit".