If you are displaying your LEGO creations on open shelves, eventually they will collect dust. However if you do a quick dusting on them on a regular basis, you can keep those dust-bunnies at bay.
The challenge of keeping LEGO dust-free is the studs on top of LEGO elements. It makes it hard to just wipe them off with a cloth, as you cannot get between the studs easily.
There are some other methods though, that are very successful in keeping your LEGO-displays clean. Check out the suggestions below!
DUSTERS: natural feather dusters or synthetic dusters that are used to keep small display-items clean works quite well for LEGO cleaning. If you use this method, I would recommend dusting off you LEGO collection once a month. Here are some you might like listed on Amazon: Feather Dusters
BRUSHES: soft, natural fiber paintbrushes that artists use are very handy to get into small crevices. I would recommend getting a large brush for general dusting of your LEGO-displays, and a smaller one to get into small spaces. Makeup brushes also work wonderful! Again; I would recommend a once-a-month cleaning with this method. You can find them at local stores as well as a good selection on Amazon: Paint Brushes, Makeup Brushes
COMPRESSED-AIR DUSTERS: this is a product used to clean computer keyboards and other small electronics. It comes in a can, and provides a tremendous amount of air pressure to expel dust, yet it is safe for delicate items. Compressed-air cans are available at office supply stores. It is not cheap (about $5/can), but it does a really good job at cleaning away dust. My experience is that if you do a throughout cleaning of your LEGO collection with this method a couple of times a year, it will keep your displays in tip-top shape. Here are some examples: Compressed Air Dusters
VACUUM CLEANERS: Some small, handheld vacuum cleaners have an option of switching over to blowing out air, instead of sucking it in. They also have various large and small attachments to control the airflow. This is a good option for very large LEGO displays, mosaics, or sculptures, however because the airflow cannot be controlled nearly as well as with compressed-air dusters, it may blow off some of the LEGO elements that are not well attached. So, I would not recommend it for small LEGO displays with delicate details. Here is an excellent example of this type of vacuum cleaner:
Metro Vacuum MDV-1BA DataVac Pro 4.5-AMP Computer Vac/Blower
WARNING! Never use a vacuum cleaner in its normal, sucking setting to clean LEGO! Your LEGO bricks will end up in the belly of the monster!
You may also want to read these sections for further ideas: