Advice from Murray Harris:
As for the study habit that has proved most helpful in my academic career, it is this. There is no better way to become proficient in Greek, to gain a “feel” for the language, and to become enriched by the theology of the New Testament than the regular memorization of the Greek text. Paste a photocopy of verses or sections of the text on to cards and carefully reflect on it as you go about your daily exercise.
via New Testament Perspectives: Murray Harris’ Advice for Learning Greek.
Of course, in addition to memorizing, reading as much and as carefully as possible doesn’t seem to hurt either in gaining “a ‘feel’ for the language.”