I'm surprised no one here has mentioned battery cycling. I have an Accu-Cycle elite and I cycle all my Rx/Tx at least once a year. That way I know what each battery capacity is and how many minutes it takes to discharge. Once a battery drops below 75% capacity, I replace it. Cycling your batteries can also perk them up a bit and increase capacity.

At the beginning of last years flying season I charged up my Shrike 40 and Hitec over night before a day of flying. On the first flight of the day I was about 1-2 minutes into the flight and I hear a beeping from my transmitter. I looked down and saw "LOW BATT" on the display. Luckily for me I was able to land without incident but it taught me a lesson. Cycle and record battery capacities often! I found the Rx battery to have about 350mah capacity and the Tx batt had about a 40mah capacity. I credit my 72mhz system for holding up and bringing my airplane back, but that's another topic.