Look into the low-power mode of whatever microcontroller you're using... I did an AT-Tiny85 project, and it burned through the battery. I then switched to low power for a while, and it draws micro-amps (!) when in low-power mode, so the battery would have lasted hundreds of times longer.

Oh... umm... the display, yeah. Maybe have an on-button for the display to power it up only when you want it?