If you are an experienced public speaker yourself and know the tricks to keep calm and deliver a strong presentation, you can provide guidance for someone else who hopes to do the same. Outside of one-on-one coaching; you can create general guides, and make short video presentations on body language skills clients can use to become better public speakers.
I created an “Every Day I’m Hustling” shirt and sold enough for the campaign to ship, but didn’t earn much on the experiment because of some not-very-effective Facebook advertising. Still, some opportunity here and a friend of mine is doing REALLY well with this. If you can create awesome designs and reach the right people with good Facebook targeting, Teespring can be very profitable.