Do not use a cloth to clean dust!
To remove dust, use a large goat hair "mop" brush. Like this: http://www.amazon.com/gp/product/B00...U5WS34J2QDMKJT
You don't want to scratch the image with a cheap synthetic brush.
For dirt, you'll need to be careful. A soft cloth will be needed.
Pec Pads are better: http://www.amazon.com/s/?_encoding=U...UVNRRUKESDESW7
This is what I use.
Liquid (water, usually) may be needed, but it can also really damage the image if not careful. A minimally damp cloth can be used on small spots, but it must be wiped away just as quickly. Even a whole second of water left of the print could be a disaster. It's worse for most non-glossy pre-1970s prints.
Repeat as needed.
NOTE: Do NOT use liquid on Polaroids!
Regardless of what you may read online, there's no one best way. Some of the methods online will ruin your images if you follow them. So beware!
Lots of solutions are sold for this, but none of really better than plain tap water.
If the damage is severe, and it looks like the print may not stand up to wet cleaning, then I scrub as carefully as possible with the Pec Pad, then scan as is, and try to work my magic in Photoshop.
More advanced wet methods can be done, but even I would not attempt them. It really depends on not just print type, but even the paper that was used.