For pronunciation, it is surely most essential to have feedback from a native speaker. If the class you took was ineffective, my guess is that another class or teacher is needed. I suppose listening and repeating to recordings of native speakers could gain some improvement. Choose anything you would enjoy, such as a scene from a movie or even favorite songs. The risk on your own is that you could develop some bad habits of pronunciation due to misperception. But if you are earnest and have a reasonably good ear, it could help. Japanese has very few syllabic sounds compared to English, so you need to contort your face and vocal chords in a lot of unfamiliar ways. It’s a steep uphill climb for many.