For Sister Missionaries in the LDS Church, it means 547 outfits! So you should get things you love and that will work for your mission and should still be your personality! Think of needs of a mission -- modest, durable, easy care, no wrinkle, mix & match and professional.
Heather Anderson is a fashion blogger who says layering is important. You can use slip extenders to lengthen a skirt, use leggings for modesty as well as warmth and think about layering tees and cute cardigans.
You can make each outfit your own. Heather suggests to keep it comfortable, fun and your identity. Personalize your look by finding things you love with your current style and find a fun way to make it work on a mission. For example, Heather loves statement necklaces, colors and prints.
Buy things that you will actually wear after the mission to save money.
For more of Heather's fashion ideas, please visit her blog: http://latterdaystyleblog.blogspot.com/