Ideally you discharge them down to 0.9v per cell (5.4v) at 20 or 30A discharge. You can do it in car but really the only accurate way is to use a discharger. You can make one easy enough, or you can buy preassembled ones that are even automatic.
I use just a string of light bulbs and a voltmeter myself, very manual, very cheap. I was not having very good performance overall with my packs so I charged them, discharged them at 20A until the pack reached 5.4V, and then recharged. I waited until the pack was fully cooled between cycles and I did this about 3 or 4 times per pack. Each time recording the amount of time it took for the pack to charge, and for it to discharge.
Doing this I gained about 30 seconds per pack, basically bringing them up to their full potential. I noticed the packs charge smoother now without false peaking and they also are more consistant and even during a race. I also know which packs are my weakest to strongest so I can save the best pack for the main.
Here are some good links that will give you a lot of info ..
Home made discharger..
Don't use a bulb discharger without either a voltmeter, or one of the relay style shut offs. You don't want to fully discharge your packs until the lights dim.
Also check out that guys site, it has a lot of good info on batteries, etc.