Salah Surah Selector app for iPhone and iPad
This app will randomly assign Surahs for you to recite in your Salah based upon the Surahs you have sected as memorised.
As such it will ensure that your Salah does not become stale, and that you do not keep reciting the same few Surahs/Ayahs in every Salah, thus stopping you from forgetting Surahs that you once knew.