The thought about filters is a good one. It has happened to me too and I get much sharper pictures without on my 18-200VR. Amazing how many people will spend a fortune on a lens only to fit a filter that gives what is costs. Sadly, that is often near s*d all.
Doing your own sensor cleaning is something I am not prepared to do, as the risks are significant. A few simple precautions will help keep the problem down to such an extent that you should not really have to do it. I do think that all of this guff about dust removal systems on cameras is over egging the pud a bit. Great idea, but overdone by the marketing dept. to make you think that you can't survive without it.
1. Think about where you are going to change lenses. If it is full of dust or airborne matter, then don't do it. Find somewhere better.
2. Hold the camera face down while changing lenses.
3. Remember to clean the rear face of the lens and its surroundings with a blower or brush prior to affixing the lens. A lot of dust can get into the box this way, so plan for it.
4. Keep the camera in a bag when not in use. Dust gets everywhere and into the body through cardslots, battery doors and so on. If you get it into the viewfinder, then learn to live with it as it can be a major stripdown to get to it. Keep caps on sync sockets, remote sockets, etc. While you are at it, empty your camera bag every few months and remove toenail clippings, pet hairs and other FOD hazards
5. Even if dust is there, you will only really see it at certain apertures and you can easily clone it out with many software packages. Some, will let you map the dust and remove it automatically, though I have not used this method myself.
6. If little Timmy is within 6 feet of your precious equipment, then poke him in the eye. Even more so, if you are not sporting filters on the end of the lens. At least, that might make you feel more cheerful.
Conan