How to dress preppy and casual at the same time?

Im an 18 yr. old guy, and I prefer to dress preppy, but I feel like its too formal to wear polo shirts or dress shirts all the time. I wear ralph lauren or other nice t-shirts to be more casual, but then they just seem to casual. Is there any way to look preppy and casual at the same time? I dont have as much trouble with pants/shorts because I can wear seersucker/madras with either casual or formal attire, but I just have trouble finding a shirt that is casual and preppy, any suggestions?