1) Use Meandering Geometry

It can decrease the space of the antenna while keeping the electrical length the same.

2) Increase Dielectric Constant of the Substrate

It will make the antenna smaller, but be careful with the performance - it will degrade as well. Some PCB materials are designed specifically with higher Dk for this reason.

3) Use Shorting Pins/Walls

Inverted-F antenna is one example of using shoring pins - it requires only quarter wavelength to radiate on the given frequency.