Hi,
There two possibilities that are known to me. The first one is that you use a balun at your output. Since there is a phase difference of 180° between each path of your balun, the common-mode, such as noise or local oscillator signal in a double-balanced mixer is cancelled. Your differential signals become at the output of your balun in-phase.
The second possibility is to use a current mirror just like in op-amps (please razavi's gray-meyer's book). In this configuration, since the phase difference between gate and drain is 180°, the common-mode is again cancelled, whereas your differential signals become in-phase.
hope this helps