Beauty isn't just on the outside.
Pick a pumpkin without blemishes or bruising. Mold around the stem is a sign of rot and should be avoided at all costs. Your pumpkin will not last as long once its carved and will smell awful! Never hold a pumpkin by its stem. Its not polite! And....it can cause rips in the pumpkin's skin. If you're not a fan of all the mess and chaos that comes along with carving a pumpkin (what is wrong with you?!) then there are also artificial pumpkins available. You carve them once and they last years. The best part is, no stringy goo or seeds to deal with.
Exfoliating and Excavation (*see carving)
Start by cutting a hole in the bottom, not top of your pumpkin. Carve this opening at an angle so the bottom can be replaced later, or leave it open and simply set your pumpkin on top of a candle later. Now scrap the inside of that thing till your arm feels like its going to fall off. If you don't have a great scraping tool, or yours breaks, (This happens to me every year. I have been told that I am an enthusiastic scrapper) a CLEAN empty tuna can is great for scraping the inside of a pumpkin.
Primer, Base, and Concealer
Place your carving pattern over the outside of your pumpkin and use straight pins to hold your pattern in place while you transfer it. Poke small holes (think connect the dots) into the pumpkin along the lines of your pattern. Once you have transferred your pattern, dust some flour over the design. This should help your design stand out and make it easier to carve the rest of your pumpkin.
Always saw/carve at a 90 degree angle and leave your cut pieces in place till the very end. These pieces prevent your pumpkin from caving in while you are carving out other ares and add stability.
http://zombiepumpkins.com/patterns.php has great patterns that you can use.
A Natural Glow
If you are using a candle, make a vent in the top of your pumpkin for the heat to escape. This heat can dry out your pumpkin and cause premature aging. Sprinkling spices like cinnamon, pumpkin spice, or nutmeg on the inside of your pumpkin will keep your pumpkin smelling great when lit and gives a festive feel. A safer alternative to using a candle is to use a store bought LED light or even get your christmas lights out of storage and use them to illuminate your pumpkin.
Here is a great idea courtesy of Martha.
Use a spray bottle and spritz your entire pumpkin to keep it hydrated. For all of your carved edges, use a cotton swab to apply petroleum jelly. If after a day or so, your pumpkin becomes shriveled, immerse the entire pumpkin in a cold water bath with just a tad of bleach. Leave it there for a few hours and when you take it out to put it on display, your pumpkin will be rejuvenated and sanitized. The bleach keeps mold and mildew at bay, as well as any creepy crawlers that may be looking to make a snack out of your jack o'lantern.
There are also special sprays and pumpkin baths available on the market designed to keep your pumpkin looking fresh.
When your pumpkin is not on display, make sure to store it in a cool place, wrapped in a plastic trash bag. This will keep your pumpkin moisturized and expand the life of your pumpkin 2-3 days. The absolute best place to store it is in the refridgerator. If you don't have room, the basement or even a downstairs closet will do.