I am a computer and electronics tech for a school district. I have worked on a number of different model projectors. The biggest problem that I see that can wear out a bulb VERY FAST is dust in the filters. When the filters get clogged with dust, the projector cannot move the proper amount of air to keep the lamp cool. These lamps run extremely hot and if not cooled properly can fail very fast. The first sign of a failing bulb is that the picture gets very dim. This is a good sign that the bulb needs to be replaced.
Another thing that can reduce the life of the bulb is leaving the projector on when it is not in use. If you are not using the projector for an extended period, shut it off. Leaving the projector in an A/V muted state does not shut down the bulb completely and can still wear on the lamp hours. These bulbs are rated for a certain number of lamp hours, so any time the projector is on, it is wearing away the hours of useable time. And at the cost of these bulbs, this can get expensive.
Most companies that sell bulbs sell the entire bulb assembly which is the bulb in an easily removeable housing that is usually made of a heat resistant plastic. One place that I use for replacement lamps is a site called my projector lamps http://www.myprojectorlamps.com
. This site will sell you the entire bulb assembly for a reasonable price. If you feel confident enough and have a bit of mechanical ability, you can usually get the bulb alone at a cheaper price and replace the one in your existing housing. Bulbs for some of these can be a bit tricky to replace in the plastic housing, so check out your assembly before going and ordering just the bulb. You may want to even try getting the bulb out of the housing first to make sure you can do it. That way if you damage the housing in any way you can then order the complete assembly.