Check out grammar books. Books, such as "Eats, Shoots & Leaves" by Lynne Truss, "The Elephants of Style" by Bill Walsh and "Woe is I" by Patricia T. O'Connor are three great grammar books which simplify grammar and approach the subject clearly and with humor.
Listen to grammar podcasts. Podcasts can explain general usage and offer in-depth and fun explanations of how to use grammar. See the Resources below for a few suggestions.
Study grammar sites on the Internet. Some grammar sites offer bullet-by-bullet points of the parts of speech and their uses. If you get mixed up on a particular part of speech, write out their usage or create mnemonic devices.
Practice grammar on a daily basis. When reading, diagram sentences, find the verbs and pronouns to practice and brush up on grammar.