+1 Sanyo Eneloop batteries with a Sanyo eneloop charger. Use those and you will have no battery problems.
Also don't to forget to check to see if you have the latest firmware version, which improves battery metering. If you don't have the latest version (1.01) you can update your firmware to the latest version, which you can download from here:
Latest K-x Firmware Update : Software Downloads : PENTAX
You can check the firmware version by holding down the 'Menu' button when powering on the camera.
This update is optional though. If you are having no battery metering issues, you may not need to do the update.