What Can I do to prolong battery life on my nikon d5000?

The only trick I know: Someone told me that after charging it, I'd have to wait for the battery to cool down for me to put it into the camera for use.

but are there any other tips?

Don't use live view if you don't have to. Don't spend all your time chimping the shots you just took. Chimping is when you view your last shot while going "ooh ooh ooh".

In the menu, set the time out periods for there minimums. That means how long the screen stays on, how long the meter stays on, and if you can get away with it, turn off menu display too.

The battery in a D5000 is good for a LOT of photos… 700 or so? Spare batteries are not terribly expensive either.

One thing I recommend is between photos, if it's going to be a few minutes, turn the camera off.

You should go with HQRP EN-EL9A Battery for Nikon D5000

HQRP Replacement Battery plus HQRP LCD Screen Protector & Black Mini Tripod
Voltage: 7, 4V ; Capacity: 1200mAh; Fast recharge rate;
Li-Ion, No Memory Effect; Integrated microchip prevents overcharging & lengthens battery life
Made from high quality A grade cells; Not OEM. 100% compatible with original batteries;
1 Year Warranty