Backup paid app from Google Play How can I backup an app I bought from Google Play so that I can install it on my phone later? 

I tried to copy /mnt/asec/[appId]-1/pkg.apk but it doesn't work.

Can someone help?
You don't need to back up apps bought on Google Play to install later.  So long as the device has your Google account registered on it, you can download and install it from Google Play at any time.