Anytime that you are honoring the life of a member of the military, it's important to look your best. Casual clothing like jeans, t-shirts or sweaters should be left at home as they will be considered disrespectful.
Both active and former members of the military should be dressed in their Class-A uniform which is commonly referred to as a Dress Uniform. A military funeral is not the time to wear your military mess dress.
Civilians are expected to dress in formal attire much like you would if you were attending a church service. Men should stick to dark colored suits or at the very least slacks, a dress shirt , and tie. Women should wear a dark dress, suit or skirt and blouse.