IAP (in-app purchases) are becoming a real problem on Android games. Here are some techniques that can help work around them.

Almost all of these require root.

get in-app-purchases for free

fast-forward the system clock

RAM editors

If you're unsure how to use these, download Cheat Engine and go through the tutorial. While there are slight differences between RAM cheat programs, the general concept is the same, and it's a little easier to learn how to do this on a PC.

backup and restore game data

This is sort of like save-states in Nintendo emulators, but you have to exit your game before you can backup/restore.

modified game binaries

websites and forums