If your laptop battery just doesn’t last like it did (or never was all that great), you might try calibrating it. This HP link gives a simple calibration procedure for Windows-based laptops … I used this procedure on mine and got drastically better battery performance from it.
Mac laptop owners might want to check this tip from Apple.
Of course batteries don’t last forever. And not all batteries are created equal. If you’ve tried calibration and it keeps disappointing you, maybe it’s time to just whip out the plastic to keep your laptop from letting you down when you most need it.