#1 Get a good understanding of German word order. If you understand where to place a word in a German sentence, you'll avoid some of the most common mistakes among English native speakers.
#2 Learn to conjugate and use regular, irregular, separable verbs and reflexive verbs by looking at logical patterns that apply to them before you commit them to memory.
#3 Once you understand the four cases and their function in a sentence, you'll distinguish between subject, direct and indirect object with ease.
#4 And with a solid understanding of the cases, you need to turn to pronouns, prepositions and adjective endings to learn how they influence the usage of the cases.
#5 increase your vocabulary by using flash cards/index cards/reference cards