Formal:
* Я практикую свой русский язык. (Ya praktikuyu svoy russkiy yazyk.) - This is the most literal and formal translation.
* Я занимаюсь изучением русского языка. (Ya zanimayus' izucheniyem russkogo yazyka.) - This means "I am engaged in learning the Russian language."
Informal:
* Я учу русский язык. (Ya uchu russkiy yazyk.) - This means "I am learning Russian."
* Я тренирую свой русский. (Ya treniroyu svoy russkiy.) - This means "I am training my Russian."
* Я практикуюсь в русском. (Ya praktikuyus' v russkom.) - This means "I am practicing in Russian."
The best choice depends on who you are talking to and the context of the conversation.