As an editor there is one thing that drives me crazy when it comes to refusing otherwise good images. Sensor dust - it can make a perfectly good landscape look ugly. Studio shots are not insured against it either. It has one of the simplest solutions, but if not dealt with can lead to a lot of disappointment.
It appears to me that while quite a few contributors do take the time to continually inspect their gear for dust, others may not even know that it's there. The simple fact is, if you own a DSLR and change lenses on it - you've got it. Point and shoots, on the other hand may suffer from lens dust (or even worse - hairs), which is not any better.
As a matter of fact, brand new cameras (DSLRs) may also have that issue. Or even worse - there's been reports of cameras such as the Canon EOS 5D coming with oil spots on the sensor straight from the manufacturer.
So, we have a problem. What's the solution?
1) No matter what you shoot as usual - periodically (at least 1-2 times a month) shoot the sky (whether blue or cloudy) with proper exposure at F8, F11 and F16 and inspect these images for dust or spots at 100% zoom.
Sensor dust generally shows up at smaller apertures (about F6.3 or more). If you're able to see it with the lens wide open, then you're in for a treat. :D
2) At least once a week clean your lenses (the front end) and filters with a designated lens-wipe or brush.
3) Keep your gear generally clean - dust off your camera at least once a week completely. A dust blower or a second lens brush might come in handy for that.
4) Avoid changing lenses in windy conditions, on the beach (unless inevitable) or with the camera's sensor pointing upwards. When changing lenses - do it quickly. It shouldn't take you more than 10sec to change the lenses and put the cap on the one that's now off the camera. Loosen the lens caps before starting the procedure.
5) Plan your shoots. If you plan your shoots so that you won't have to change lenses back and forth this can save you lots of trouble. It's best to have a backup camera for that, but in case you only have one body - shoot with one lens first, then when you're really done - change to the other.
6) If (and eventually you will) you discover more than 4-5 dust spots on your periodical dust check, use a dust blower to try and fix that. If this doesn't help - get some designated wet cleaning and apply them on your sensor. Note that some manufacturers have warranty conditions when it comes to sensor cleaning.
So, if you're not feeling all pumped up and confident about it - send your camera in to an official service every six months or so for a cleanup. If you shoot less - once a year might do it just as well.
Lastly, whatever you do, and however you do it, make sure it's had an effect. Repeat the sky shooting exercise after you're done cleaning.
If you only have a few spots - please, always remove them carefully with some editing software such as the GIMP (totally free, so no excuses!) or Photoshop.
And now because you were caring enough to read through the whole article, here's a little fun fact for you. Even the satellite shots of Google Maps have sensor dust.
Yup, it's that big of a problem. Click on this link to see the dust.
They've totally ruined the otherwise nice looking Rotterdam seaport. Note the multiple small dark spots throughout the image... bad bad.
So, shows us you can do better than that! Then, when your images go online you can show off before your friends that you've outdone Google Maps in terms of image quality. ;)