Laptop performers actualy have the advantage in low light situations. Our chosen tool actually comes with its own built-in light... the computer screen/monitor. Even with that said I have been in some places to perform (DJ booths are a great example) where other than your immediate computer keyboard, you can't see a thing. Drop something? Have to check a cable connection? Having a flashlight handy could save you an embarassing moment. Even just a small keychain flashlight may save you. You could always keep it real by wearing a miner's helmet but that could be overkill...
At a minimum, remember that your phone can act as a flashlight in a bind. There are even flashlight apps for the iPhone that basically just white out the screen to maximize the effect. If you're in a low light situation and crawling around looking for something, remember your phone!
Use the Wall
Even though your set might be short enough that your battery should power you till the end... use your power adapter and outlet. Many periphrials you may use (like external hard drives, midi controllers, and audio interfaces) suck all your power. Depending on your powersaving options, your screen is likely to turn off, hard drives spin down or worse at the wrong times. Don't run your show off your laptop battery!
At a minimum, I also recommend limiting the amount of bus power you're using for your controllers on stage. I would much rather keep the laptop focused on the audio stream it has to play back than powering the lights on my USB controller. Some USB controllers I've had would randomly power down or blink on and off when using bus power (especially with a bunch of other devices plugged in) but work perfectly fine when using a dedicated power adapter.
Just avoid the whole thing if you can and plug both your laptop and your external devices into the wall.
Disable Power Save Options
Power saving is a great thing. If you're like me you leave your computer(s) on 2/47 though I'm not actually using them all 24/7 ( usually :P ). Power saving options help control your energy consumption when you're not actually needing power. Good for the environment and your electricity bill.
However for live or "mission critical" use, power saving options can cause lots of problems. Ever had a laptop refuse to "wake up"? I have. Not so good on stage. Turn off all the power saving options when you play but try to leave them on when you're not.