I typically don't carry cables on me, but that's because I have a stickshift, so if I kill my own battery, I can roll start the car.
Just remember these things when jumping a car and you'll never have problems:
1) Start with both cars completely off, key out of the ignition, no headlights on, no stereo, nothing, and all dome and map lights set from DOOR to OFF if possible. Keep them in P (automatic) or N (manual) with the parking brake fully set.
2) Positive post of jumpee's battery
3) Positive post of jumper's battery
4) Negative post of jumper's battery
5) METAL GROUND of jumpee's CAR (do not use the negative post of the jumpee's battery, ever).
6) Start jumper's engine, wait three minutes, and hold the throttle at 2000 rpm if possible.
7) Start jumpee's engine, let it run for a minute in-case it wants to sputter and die out.
8) Disconnect the metal ground of jumpee's car
9) Disconnect the negative post of jumper's battery
10) Disconnect the positive post of jumper's battery
11) Disconnect the positive post of jumpee's battery
Done.