To get the best from your rechargeable batteries :
Buy a good charger. Make sure you get a “smart” charger, one that electronically monitors the recharge process and prevents overcharging.
Remove the batteries when the recharge is complete. Unnecessary time on the charger means more “trickle” power is used to keep them charged.
Don't regularly discharge your batteries completely in an attempt to make them last longer. Complete discharge can actually shorten their lifespan.
Store your NiMH batteries at room temperature in a dry location. Excessive heat can definitely damage your batteries and cause them to lose charge faster.
Consider using low self-discharge NiMH batteries, which don't lose power as quickly when sitting on a shelf. If your rechargeables are ready to use, you're less likely to fall back on the throwaways.