How to clean a filter or lens: No it is not a dark art!!
First up... is cleaning at home where more care can be taken.
Blow that dust off with a blower. Next take a good quality micro fibre lens cloth ( sorry but lens tissue really went the way of the dinosaurs) and if the glass is not really that dirty a huff of breath or two and a nice gentle circular motion with the cloth should see you right. Another quick blow and then screw a filter on that front element and your lens is protected and clean.
If you have oils or salt water marks etc... on the glass then some multi coated optics will require the use of a fluid to remove them. A good lens fluid will work in this case, such as Opticlean by B+W.
Here's how to use a fluid correctly. When using liquid cleaners you will need to spray a small amount of the fluid on the cloth ( if you over spray wait and let the cloth dry a little). Now with your slightly damp cloth in hand with firm pressure and a circular motion wipe the glass surface you will see a haze or fog develop this is the liquid drying ( like a polish on your car). All that is left to do is polish off this haze with a dry part of the cloth until the residue is completely gone. Bingo you should have a crystal clear piece of glass. If any cloudy haze remains just polish again to get into edges you might need to put a the point of a finger under the cloth.
Removing oil from some filters such a Hoya brand needs more drastic action, for these the following ritual works.
.Wash your hands well then place a drop of good dish washing detergent on the glass and rub this over the glass with a finger tip and rinse well under warm water, polish dry with a micro fibre cloth. Works a treat!
Second scenario... ok now your in the field...
And your filter (or lens though it should have a filter) is covered in dust and crap... no micro fibre cloth to hand... ( tut tut... remember the motto be prepared!) In a pinch you can use a clean T shirt edge or other item of soft clothing. Just don't press too hard and blow off what you can before you start. I am guilty of doing the above far more often than I care to admit without any consequences so far... In fact when in the field shooting I leave the lens caps off my lenses as then I am always ready to shoot... but I always have a filter on. One last tip don't get too obsessive about cleaning unless your lens is ultra wide there is little chance of a bit of dust affecting your images. Note to rangefinder owners make sure you keep that front viewfinder window clean it really gives a much clearer view! Happy snapping.
Another helpful guide from the team at Mainlinephoto!