Display
Similar to phones, laptop screen is also a power guzzler. Keep the brightness as low as possible. Also, If you also have keyboard backlighting, turn it off.
External devices
Any external device plugged in the USB port constantly draws power from your laptop. Removing them after use conserves battery.
Overheating
Overheating leads to the internal fans to run faster - which in turn consume more battery. Use a laptop cooler to keep the temperature at bay.
BatteryCare
This application records the battery discharge cycle and also shows a detailed battery status including consumption, manufacturer, wear level etc. It also shows CPU and hard drive temperature to indicate any overheating.