The first thing to remember is that, while there are a number of free flashcard apps available, they tend to limit the number of vocabualry sets you can download. Therefore, I suggest you buy an app for your mobile device, allowing you to download as many sets as you want. Having said that, most Apple users are lucky because there is a great free app for them.
Apple users should get the Quizlet app (specially designed by Quizlet for Apple products) which is free from iTunes.
Quizlet are in the process of developing a free app for people with a smartphone using the Android operating system but, for now, you will have to buy one. I recommend an app called Quizard. Many students have told me that it works well with Quizlet. It is available from Google Play for ¥237.
I hope this information helps. Please let me know how you get on with these applications.