Make sure, once you have learned a language, to turn part of your life into a world operating in that language. Choose something - news, movies, books, internet - just pick something. Do that thing only in your new language once you can.
Once that becomes a habit, what you have learned you will not forget. And you will get better over time as well as your brain painlessly absorbs and deeply programs in the target language.
Second most important part - It takes a lot of work and consistency. Say that with me - consistency. Show up every day. If you don't, you will fail. That's one of those terrible truths about learning, and it is especially true about learning a language.