how can i increase amplifier's linearity?

First of all you should understand where your limitations comes from.
For example, in a cascode configuration, the gain is a gm*Zload.
If you have a large Zload, most probably the output voltage swing is clamped, but also you can have limitations from the equivalent Gm of the transconductor.
Giving an accurate answer is very difficult without with no informations on the amplifier.
As a general rule, in common emitter (or source) topologies adding an emitter degeneration will increase linearity, reducing gain.
I hope it can help.
Mazz

One trick is to use big DC power supply bypass capacitors (much bigger than you would normally think you need). For any type of multicarrier modulation, there can be huge differences over time in the DC current draw requirements as the individual carrier currents peak or valley, and most power supplies can not handle them without drooping. A capacitor bank can help a lot.

Low-noise amplifier stability concept to practical considerations

just type it at google box.... hope that it useful to u...

I think what biff44 said is like the trick used in the attached application note. Hope it helps.

CMOSBJT

Yeah~ This trick is called "Low frequency trapping network" :)