Since installing Mountain Lion I found that my battery life dropped to about 1 to 1.5 hours! I also noticed that the fans on my MackBook Pro were almost constantly spinning (even when I had almost no applications open).
This is not an uncommon problem, it would seem that many others have had the same struggles. See these threads for some posts on this topic.
So, here's what I did to fix the problem:
Simply open the Activity Monitor (in Applications/Utilities), when it is open click on the 'My processes' tab at the ton, and choose to show CPU cycles (it is a heading that you click on between 'User' and 'Threads'). You should be able to see what processes are using up most of your CPU cycles.
I found that Microsoft Sync services and iCal External Sync were constantly spiking to the top of that list.
The point is, if you see an application or process that is not necessary that is hammering your CPU, disable it.
So, I opened up Entourage 2008 (I have to use Entourage because one of my clients runs an old version of Microsoft Exchange (2003), this old version of Exchange doesn't work with Apple Mail, Calendar, or Address book. It also doesn't work with the new version of Microsoft Outlook 2011 - sadly!) In Entourage, click on Entourage, Preferences, look for Sync services on the left and then make sure you don't have any sync services enabled. This will mean that you won't sync your Entourage calendar, address book, or notes with Apple's products. I suggest that you export and sync these with Gmail from time to time, and then sync Apple's services directly with Gmail.
The second thing I would suggest you check is what items you have set to automatically open up when you login to your mac. I had the 'Growl' agent installed, and since I have an Android phone in addition to my iPhone 4 (I also carry a Samsung Galaxy Note), I had Kies and the Android file transfer agent set to automatically load). When I deselected these (in addition to what I did with sync services above) my battery life was so much better.
Here's what you do to check and disable startup items.
First, open 'System Preferences', then click on 'Users & Groups', now make sure your user account is selected (it should be under 'Current User'). Then click on 'Login Items' (this is not 'Login Options' on the bottom left, it is the tab next to 'Password'). You will see a list of programs that are set to open when you log in. As mentioned Kies, the Android file manager, and the Growl agent were among the programs listed. Click on each program you want to remove and then press the '-' button (don't just deselected it). That will stop it from starting up.
Well, I hope that helps you! It helped for my batter life. Your results may vary. If you find another solution please do drop a note in the comments section below and let me know.