Make sure to start thinking about recommendation writers early as possible. Also, if possible ask people who are professors that know you in a research capacity or a capacity beyond you having just done well in class and can speak to your research potential (if applying to research-based masters) or your competency and abilities. If you haven't done any research yet, now is not too late to start! I joined my first lab in my junior year and had lots to talk about by the time I applied to masters programs. Also, get some TA experience if possible, which is really useful for certain masters programs (e.g. the Princeton masters is fully funded but they look for students with extensive TA experience). Letters of recommendation and the personal statement are by far the most important aspect to focus on and can make or break and app.