Rectangular headdresses, which use to be quite popular, aren’t seen much anymore and hairbows, of the larger type, seem to be the new accessory of choice.
One thing I did notice about how rectangular headdresses’ design progressed was that they started being designed with more and more embellishment, which after a certain point, embellishments can no longer be added and if the item has too much embellishment, it can look gaudy.
My personal preference has always been round headdresses, smaller side hairbows and hair combs/corsages, whether flowers or bows. All of these can be quite flattering with a variety of hair styles, types and lengths.
Smaller side hairbows – most hair lengths
Round headdresses – Short (though on the long side for short) to long hair
Hair combs/corsages – Medium to long hair (though petite sized combs are good for short hair too)

See Dollpart’s entry on hair accessories