It's easier than you think.
Take one song you really like.
(1) Listen to it while reading the lyrics.
(2) Make sure you understand all words/phrases, including their original context and cultural references if any.
(3) Sing this song a few times.
In a couple of days you will be able to remember 10-20 new expressions without any flashcards or memorizing.
There are a few catches though:
(1) You've got to love the song and feel like singing it (doing it just for practice does not work)
(2) You've got to find the way to use these phrases in real-life communication later on, so you won't forget them.