This site has some great tips for getting a dog use to wearing bows and topknots.
Top-Knot How To's - The Diva-Dog Bowtique. It suggests starting just with a band and when you add a bow, start with the baby bows that are single looped bows that aren't very heavy. Make sure sure you aren't putting the band in too tight, your dog should be able to feel the band. Also, this site shows how to put a double band in to lock the bow in place.