I went to SS last year.
Besides what other people have said already which i totally agree with (make friends, get as fit as you can):
-Bring a small bottle of laundry detergent. Sometimes you are made to change into your PT uniform more than once a day and you only get like three shirts. I washed my shirts in the sink and cycled them out which was very helpful, my roommates also found it very helpful.
-Bring bandaids and blister bandaids. Sometimes your feet arent used to the type/intensity/frequency of workouts you will be put through.
-Make an effort to learn your knowledge. I showed up to SS already knowing the phoentic alphabet, the core values, and some other basic AF information which ended up being on the packet of knowledge they gave us. Its that much less you have to cram the night before MilEx.
-Bring a lot of plastic bags. This helped me because you or your roomate always seem to need another one for laundry, wet clothes, dirty clothes, shoes, toiletries, etc.