Inverted F antenna bandwidth

Currently I am designing dual band IFA antenna for GSM900 and DCS using CST. But I am finding it really hard to get the required amount of bandwidth. Can anyone share techniques by which I can increase the bandwidth ? Thanks in advance.

Techniques to increase the Bandwidth for PIFA:
-The e increase in substrate height and decrease of substarte Er can be used to increase the bandwidth of the PIFA.
-Bandwidth is affected very much by the size of the ground plane. By varying the size of the ground
plane, the bandwidth of a PIFA can be adjusted. For example, reducing the ground plane can
effectively broadened the bandwidth of the antenna system. To reduce the quality factor of the
structure (and to increase the bandwidth), can be inserted several slits at the ground plane edges.
-Bandwidth of PIFA is inversely proportional to the quality factor Q that is defined for a resonator:
Q = Energy Stored / Power Lost
-Use of thick air substrate to lower the Q and increase the bandwidth.
-Using parasitic resonators with resonant lengths close to main resonant frequency.
-Adjusting the location and the spacing between two shorting posts.
-Excitation of multiple modes designed to be close together or far apart depending on requirements
-Using Stacked elements it will increase the Bandwidth.