Not sure if there is a guide on here yet but i'd thought I would share with everyone :]
It's supported by google and itunes and works very well.
Step 1 : Go to the market place and download the app " app trailers " it has the picture of a FilmReel and is made by AppRedeem Inc.
Step 2 : Run the app, and be sure to allow push notifications or your points will not go through.
Step 3 : Click the bottom right tab called settings and register your account. Use my referral code "murmasa" this will give you easy extra points. You can do this by going to the video tabs and clicking on " Enter Bonus Code ". You will immediately get 90 points.
Step 4 : Go to the videos tab and watch the 30 second trailers ( I will usually just click watch trailer and do something else )
Step 5 : Be sure to download the app and try it for 2 minutes, App Trailers will notify you when it will count. This gives about 100-200 extra points which is a lot.
Step 6 : When you have saved up enough points Itunes users will use points to get an Itunes cards, ( Google Play Card for the Android users )
Step 7 : Use the card to purchase your gems in game!
Done ! It takes a small amount of work but after all it is free ;].
P.S I would avoid doing the big point offers because most of them will send spam to your phone. The netflix and gamefly ones are legit though.
If you have tried it and love it as much as I do please share with us!
EDIT : IF YOU CAN NOT FIND THE ITUNES CARDS BECAUSE YOU ARE OUTSIDE THE U.S THERE ARE ALTERNATIVES
You can buy the Ebay Card / Amazon Card / Paypal Card to purchase the points straight from Itunes, or you can buy an Itunes card!