**if you don't want to do all the research and just want a simple "spoon feed me" answer...just do #3 below***
Some batteries suffer from "memory effect"...others don't and can just be topped up. NiCd's sould be fully used, then charged back up....LiPo's are ok to just "top up". Check the Battery FAQ and Google to find out more on your specific type.
If you don't end up normally running down your battery with normal use...and if you want to see that your battery is fully discharged...then you can do a couple of different things.
1. Get a multi-meter and take a reading of your pack...each cell of the battery has a "bottom floor" lower voltage that it should not fall below. It varies for different types of cells. For example, for one type of cell it might be 1v...for another it might be 3v. Multiply the number of cells by the lower voltage value and that's your ballpark number. Ballpark in that you probably won't be able to read the voltage of each individual cell in a shrink wrapped pack...and you don't want to over drain any given cell. Check the battery FAQ and Google for your batteries specifics.
2. Get a smart discharger...basically a device that you set that will drain your battery to a set limit. You still need to know what that lower limit is to set it properly. Again...depending on the unit it may not be able to "see" more than the total pack voltage. Check the battery FAQ and Google for your batteries specifics.
3. Get a good smart charger that has a lot of bells and whistles. Most good smart chargers will setup a different charging profile and meter the pack while it is charging it so as to deliver the best charge and condition the pack as you cycle it. Most all of them have discharge/charge option (single cycle or multi cycle), where the charger will discharge your battery (while monitoring it) and then charge it back up.
A plain red/green light charger (cheap and often look like a blue brick) is better than a "dumb" brick charger...but they're not very good at all in the grand scheme. Get a digital charger that will actually display what is going on with your pack (how many mAh it drained/charged, etc...) and will let you fine tune the charging amperage. Charging slowly will take longer, but is better for the longevity of your pack. Good smart chargers will also have time-outs/cut-offs that'll help avoid boo-boos if you misconfigure the settings.
Now, power is power...so if you can run the numbers, figure out and understand what is supposed to be happening, sit there with temperature probes/multimeters/stopwatches and whatever....you can do all the "smart" charging/discharging yourself. Don't know about you...but I just want to figure out my initial settings, plug in the battery, push a button and let it do it's thing without killing my battery. A good digital smart charger pays for itself not only in keeping your batteries (they've got to be good batteries to begin with...you can't buy shit batteries and expect them to rock all year long) running well for as long as possible...but also in the peace of mind and convenience factors. Nothing sucks more than getting all set for a game and finding out your battery isn't charged when it's time to hit the field...you look like an idiot.