i firswt used a bottle brush to clean the bottles (squirt a little soap in the bottle and fill with hot water) i use the soapy bottle brush to put in the suction cup part(i keep the horn all together, just take off the flap) and swirl it around to get off any oils or things that get on to them. then i run the hot water through it to get some soap and water through the whole thing. i also run them throught the dishwaher a couple of times a week to sanatize them.
I recently bought an Avent microwave sterilizer and sterilize my parts once or twice a week. Very convenient (vs. boiling water). I wash in soapy water with a bottle brush after each use. Also, a friend (who is also a nurse) reminded me I should run by bottle brush through the dishwasher often and replace it regularly -- she said lots of bacteria can build up on the brush.