In many cases, regular dusting is the only cleaning your picture frames will need. If you dust often enough to keep debris in the air from adhering to the frame, there is often no reason to dismantle or even wet-wipe your frame. The exception to this is frames displayed in the kitchen or dining room. Because cooking foods releases grease particles, it is likely your frame will need to be wiped down once in awhile no matter how well you dust.
If you are doing more than dusting, be sure to remove your art from the wall before you wipe it down. By pressing - even gently - on the top of the frame, you are putting strain on the hangers and/or the wire, which could cause them to weaken and break over time. When you take the frame down, spend a moment examining the hangers to make sure they are still well attached, and the wire hasn't loosened.
If you need to use water or cleaner, be sure to spray onto your cloth, not onto the frame - this will prevent you from using too much and having it seep into the frame (in the case of wood frames).
A dust cleaner can be used on lacquered wooden frames (those with a smooth shine) if they are quite dirty or dusty. A slightly dampened cloth also works well to remove any caked-on dust. For wooden frames with ornate designs or small grooves, use a cotton swab or a toothbrush to - gently - remove dust from the hard-to-reach spaces. If your wooden frame is not varnished or lacquered, do not use a dust spray on it - the unfinished wood will absorb it and cause discoloration.
If your metal frame is gold-colored, silver-colored, or aluminium, a slightly damp cloth should work well to remove any dust. If it is particularly dirty, a small amount of gentle soap can be used as well. A cotton swab can reach any small recesses, but a toothbrush is not recommended. For any uncoated frame - such as solid pewter or sterling silver - use a silver cleaner and follow the instructions on the bottle.
If you don't know the frame material, test out your cleaning method of choice on the the back or bottom of the frame where the finish is the same but any potential damage won't show.