Here is the method for constructing a 50 foot USB charger for your Raspberry Pi.

Step 1: Assemble Your Supplies and Tools

Supplies

50.5 feet of 24AWG cat 5 cable (Not braided)

Shrink tubing (to fit)

Solder

5.9V power supply

USB extension cable

Tools

Soldering Iron

Wire Stripper

Heat gun

Step 2: Prepare the CAT5

Measure 50.5' of 24 Gauge CAT5 cable
On both ends of the cable, strip and solder all four solid colored wires together.
On both ends of the cable, strip and solder all four striped wires togther.

Step 3: Solder the Pieces Together

Put two sections of shrink tubing (long enough to cover your solder-job) on your cable now before proceeding so you have a nicely finished cable when done.
Pick either end of the 50' cable. Solder the striped set of wires to the red (or voltage) wire from your power supply (snip off the existing connector). Solder the solid set of wires to the black (ground) wire from your power supply.
On the other end of the 50' cable, solder the red wire from the clipped USB extension cable to the striped set of wires. Solder the black wire from the clipped USB extension cable to the solid set of wires.

Step 4: Seal It Up.

Insulate the sections with electrical tape first, then shrink the tubing with the heat gun, and tape with more electrical tape as required.

Step 5: Why It Works

Using this handy-dandy voltage drop calculator we can see that a phenomenon called voltage drop will decrease the amount of usable current as energy travels through the 50 feet of copper. If we had simply used one pair (out of four) in the CAT5, we would have ended with a pitiful voltage of 1.03v once your Pi is plugged in on the other end. Poo.

By wiring all four pairs in the CAT5, we provide four times the medium for the electricty to travel through. BEHOLD, the end voltage of 4.88v -- perfectly within range for the Pi, even while drawing as much as 2 amperes.

In the end, you could probably get away with a little more length and still stay in spec, but your wire/power supply might get a little toasty.

Use the calculator, as you should have all the information you need to calculate the voltage drop. You may even be able to make a longer cable with a beefier power supply, but overheating in the cable and at the power supply could become an issue even if the correct voltage to power the Pi comes out the other end.