I use a low pressure bug sprayer. The kind you get for ten bucks at Home Depot. I actually have two. One has a soapy mixture of Simple Green and water. The other just has water. I use the soapy one first and then rinse with the water only one. The low pressure is high enough to get the dust and mild dirt off the bike.
If you've been riding in the muck at Burning Man then you'll need stronger measures.
I just use water-damp microfiber cloths (overkill, I know, but I have a ton of them for my car). My bike doesn’t get too dirty as I seldom off-road, but this does a good job getting the basic dust and dirt off, and the bug splatter which tends to accumulate on the underside of my Down Tube. The same works for cleaning my mirrors.
I’ll use a separate clean microfiber with isopropyl alcohol on it to wipe down the rotors, and after that is done, I’ll use the same cloth to wipe off any grease which may have gotten on the frame (on the chain stay, etc).