I am glad you brought this up. I believe:
1- Love. We need to love what we learn and we will not forget it that easily. (I heard) Study shows that if we attach an emotion to a memory, we tend to remember it forever. That is in fact why women tend to remember everything; because they mix daily life activities with emotion.
Now if we love learning a new language, not because we have to learn it to pass a test or something, then it fades out slower.
2- Teach. we need to educate others using the knowledge we just earned. If we teach our English language skills to others, we deliver that knowledge from short term memory in our brain to somewhere else which remains longer. I must admit you are the best at this part. We can all see you as the most generous member of this site.