Reducing hiss from mini cables
 

I have a Canon T2i and a Rode videomic, I'm making a short film this weekend, and I wanted to use the Rode on a boom pole, but I've heard that if I put 15 to 20 ft of mini cable I'll have some hiss or interference.
Is this something that can be reduced or taken out in post?

Or do they sell cables that won't do this?

It all depends. An unbalanced cable is not inherently noisy or lossy. But it doesn't have the noise rejection that you will find with a balanced cable. "Rejection" is the operative word there ... if you're in an electrically noisy situation, like near power lines carrying high currents or close to flourescent lamps the unbalanced cable may pick up hum and other garbage. But if you're in a relatively quiet area electrically, you might be fine. Make sure the cable you get has adequate shielding - the cheapest ones often don't.

Extending an unbalanced audio cable makes it more vulnerable to noise and interference but won't necessarily guarantee you'll have a problem. It depends on where you're working and how much RF signal is present that can bleed-in, as well as the quality of your cables/connectors and how much strain you're putting on them. I'd keep them as short as you possibly can if you have no other choice but to use your VideoMic. Rode makes a cable specifically for this purpose but it's only 10-feet long.
If at all possible I'd use a different recorder from the T2i. There's lots of discussions regarding this subject, but at the minimum it will free up your camera physically and depending on the recorder will usually give much better audio recording.

There are a few problems here:

1) The Rode Videomic doesn't have a particularly strong signal.
2) The weak mic output over a long distance of unbalanced cable is susceptible to interference from power cables, cell phones and other RF sources.
3) The T2i has automatic gain control. You need to feed a strong, consistent signal into one channel to defeat it. juicedLink and Beachtek offer preamps with this feature. If you're crafty, you can rig something yourself.
4) Canon's cameras have noisy preamps. If you feed them a weak signal and add gain in the camera or in post, you will end up with noise.

For recording into the T2i, I recommend a juicedLink DT454 preamp. It has clean gain, an AGC killer, a headphone output, meters, and both 1/8" and XLR inputs.

I'd also recommend a mic with an XLR connector. A Rode NTG-1 or NTG-2 or Audio Technica AT875R will do the trick.

OR - use an external recorder. Since you'll have a boom operator, he/she can operate the recorder. But you'd still be better off with an XLR mic at the end of the pole.

A high-end (expensive) recorder will sound way better than recording into the camera. A mid-priced recorder like the H4n or DR-100 will sound only slightly better than recording into the T2i through the DT454. (The 5D2 has manual audio control. The DT454 and 5D2 gives the mid-priced recorders a run for their money.)