You probably don't need to clean it unless you think it presents a problem of heat buildup. If you do wish to clean it, use compressed air (or canned air) and a vacuum cleaner as above.
Make absolutely sure that you do not allow liquid "canned air" (which is usually actually refrigerant) to come in contact with the glass tube. You can easily crack it, especially if it's still hot from operation, and then you've got a useless monitor. Compressed air (from a compressor) presents less of a hazard in this regard. You can also wipe things down with a cloth, but do not use anything wet or damp on the back side of the tube (which generally doesn't need cleaned, anyway, as it being dirty presents no problem): the water can remove the "dag" coating which leaves you with a pretty useless tube.