RFI/AC Filter Rejection for Instrumenation Amplifiers
I would simply place a capacitor across the InAmp's inputs, or rather a couple of them in parallel, like for instance 10 nF, 100 nF and 10 &micro;F. These should short-circuit any RF signals and 50 Hz noise picked up by the cable. Ignoring the 10 &micro;F for HF (it won't perform that well at high frequencies), 110 nF is 0.14 &Omega; at 10 MHz, which should take almost all energy out of it. The 10 &micro;F should take care of lower frequencies.

In theory the parallel capacitors should be just one larger capacitor, but their impedance rises again at higher frequencies due to ESL (Equivalent Series Inductance), and the poles are different for different capacitances, like this graph shows:



By placing different capacitances in parallel you take advantage of the poles for each of them.