Never ever use the transpose button on the keyboard!!!!! You'll never learn if you do. And once you use it, it's a slippery slope.
For technical skills:
I would play it again and again. Then use it in songs that I know. Then finally, use it in EVERY single key.
For songs:
I would actually play as it was intended originally. That way, not only I familiarise myself with the song, I would also learn what the writer intended (what actually goes in his head - some of his techniques in chordal progression may be helpful to me in the future). I would then play the song (original) in EVERY key (white and black, from C, Db, E, Eb etc to B). Then I would play it as I intended - in every key.