Android - How do I install an apk as a system application?

Found these instructions to install PowerManagerPro into /system/app, just modified them a little. (Note: first install the app into /data/app, like you would any app, and you need a rooted device.)

Open a Terminal Emulator app and execute the following command to copy the application to the sdcard:

su
cp /data/app/your.apk /sdcard/your.apk

If you can't find the application in /data/app try /data/apps-private/. Then, un-install your app from the device.

Execute the following commands in the terminal emulator to remount the /system directory as read/write and to install the application into the /system/app directory:

su
mount -o remount,rw -t yaffs2 /dev/block/mtdblock3 /system
cp /sdcard/your.apk /system/app

If you prefer to use adb from your computer, execute these commands:

adb remount
adb shell cp /sdcard/your.apk /system/app