How do I make my bicycle as ugly as possible without affecting the riding qualities?
popular techniques in approximately increasing order of cost include:


dirt: mud, road grime, let them cover any shiny parts
stickers
pad bits of the frame with old inner tubes held on with duck or gaffer tape
apply spray-paint in ugly or mismatched colours (optionally after keying or stripping the original paint). Then put stickers over the top
cover any desirable decals or etched logos with paint or stickers
strip some of the paint off entirely and re-paint in mismatched colours
even more stickers
strip some of the paint off entirely and, if steel, allow to rust a bit before clear-coating over the rust

you can also clear-coat over stickers if they're not that weatherproof
cosmetic damage to non-functional parts (scuff or cut the saddle, scuff 
levers or bars, choose the brightest bar tape you can and get it really dirty looking)