I took two years of Japanese when I was in university, and I've forgotten most of it by now. :lol
Repetition is key. The best way I learned to do flashcards was by making a game out of it. Try to set goals and then see if you can beat them, that sort of thing. I also liked trying to read children's books in Japanese and translating them. This is tough though - you'd be surprised how little you know about the language until you realize how much more kids in grade 1 know than you.
In the end though, there is no substitute for talking to other people in Japanese. Reading and writing the language are different than speaking it and listening to it (listening was the hardest for me). If you have family members here who are Japanese then talk with them in Japanese as often as possible. I would also advise looking online for groups who get together to do just that.
I don't know where you live, but if there are YMCAs or Open Door Societies where you live, those would be good places to find out if any Japanese immigrants need English tutoring, and instead of them paying you you could spend one hour teaching them and they could spend one hour teaching you. You could also check out local universities or schools where Japanese is taught - there were always anime clubs and stuff like that where people would go watch movies and practice their Japanese.
In any event, I really encourage you to stick with it! It can be really tough at times but being able to think and speak in another language is a really good way to keep your mind sharp.