Get the most out of your laptop battery
Over time, your battery holds less and less charge, meaning your laptop doesn't last as long between charges. Apple claims that their batteries are designed to retain 80% of their original capacity after 300 cycles.
You can check how many cycles your battery has done by looking in System Profiler. You can find this by clicking on the Apple menu in the top left and choosing About This Mac. In the window that appears, click the "More Info..." button. In the sidebar of System Profiler, click on Power to bring up all the details about your battery. The interesting part is the Health Information. (Note that the stats shown below are for a fairly new battery).
Charge remaining (mAh): 4940
Fully charged: Yes
Full charge capacity (mAh): 4940
Cycle count: 21
If it says "Check Battery" under Condition, there is likely a problem with your battery. The good news is that if the cycle count is under 300, Apple are likely to replace the battery for free. If your laptop is under one year old, they will almost definitely do so. However, before seeking out a new battery, make sure to completely charge and discharge the battery at least one to check it is not a temporary problem.
If your battery is in good condition or is starting to show signs of aging, you will want to make sure you get the most out of it before you have to replace it. Here are a few tips to prolong the lifespan and battery life of your battery.
The best way to use your laptop on a day to day basis is to keep it fully charged as much as possible (i.e. plugged in), but run down the battery fully at least once per month. This is because it is important to keep the electrons inside moving occasionally. Doing this will make sure that your battery will continue to recharge to the maximum capacity for as long as possible.
To get the longest possible battery life, you should use your laptop between 10° and 35°C (50° to 95°F). More importantly, when storing your laptop you should keep it between -25° to 45°C (-13° to 113°F) even if you are not using it.
If you are going to store your laptop for a long time, Apple recommends you remove the battery from the laptop and store it at 50% charge. If you leave it fully charged or fully discharged you may find the battery loses a lot of capacity.
A simple method that can make your battery last a lot longer between charges is to tweak your System Preferences. Firstly, go to the Energy Saver Pane. Here, make sure the optimization menu is set to Better Energy Savings for when the laptop is powered by the battery. This will put the hard disk to sleep whenever possible, and reduce the laptop's CPU performance.
One trick that I often use if I am trying to squeeze an extra half an hour out of my battery is to reduce the brightness of the display. The simplest way to do this is to use the F1 key (F2 will increase the brightness again). Dim the screen to the lowest comfortable level, which should be pretty low especially if you are indoors.
You might also want to eject any CDs or DVDs you aren't using, quit as many applications as possible, disconnect any unused peripherals and turn off Bluetooth and Airport.