Maybe you should give us more details about this antenna you wanna design.
there are many ways to enhance the bandwidth of antennas:
you could enhance the thickness of antennas, the bandwidth can increase with the antenna becoming thicker.
you also could place a defective ground on the back of antennas, but you must know, in some case, defective ground can not meet the demand of customer, cause the back radiation from antennas often effects the circuit behind the antennas and also does hamful to brain( espacially head-wear antenna).
Furthermore, you could use some special structure to realize bandwidth of antennas, for example u-slot and some kind of thing like that. As my experience, 5% bandwidth could achive without much difficulty (working at 2.4GHz, so 5% is 120MHz).
at last, a good book for you, compact and broadbadn microstrip antenna, there are a lot good examples in this book, you should find it and download it by internet.