All of the aboveis good advice. Know your batteries capacity and the usage of your plane. I have an easy method that has worked well for me.
Cycle the battery a couple of times and fully charge it. Make a resistor lead for your volt meter(I use 20 Ohm 10 Watt). Test the voltage of the fully charged battery after a few seconds of load when the voltage stabilizes. Leave the load on it and record voltage every 5 minutes until the voltage falls rapidly. If you graph the results you will see the voltage drops very slowly over its capacity, and falls off at the end. Back up 10 or 15 minutes from where it begins to fall off and mark this voltage on the pack. Test the voltage every flight, and Do Not take off when the voltage reaches this point.
You will find out very quickly how many flights you can get out of a fully charged pack. Remember, Nicd batteries have a memory, and they do not like to be partially discharged and recharged. You can charge at the field if you need to get a few more flights, bud it is better for the battery if you don't. From the sound of it, you are not even using half of the capacity of your batteries.