It really depends on your noise source.
Keep analog traces away from digital ones, have a very, very clean analog reference (caps, chokes), and remember to take several samples before offering a value to the higher layers.
Also, remember to have caps on the supply pins of all devices, I header the sharps are picky with current when it comes to the first pulse (not sure if IR or sonar, though)
by the way, I don't know that filter frequency, but it must be calculated. a low pass filter will also limit the available frequency of your sensor (you won't be able to take 1000 samples per second, but only 10. Even if you take 1000, only 10 times they will be updated. In theory, at least).