I cant offer any advice for the convocation photos. But i can offer advice for the yoga photos.
A central theme in most good yoga pictures is nature and specifically the color green. Poses like "Lotus" "crane" "lord of dance" "forward stretch pose" and "tree" are very effective to displaying skill and difficult but beautiful poses. Also work with the angles in the body and the background. I hope this helps
Canon EOS 40D; EF 300mm F4.0 L IS USM; EF-S 10-22mm F3.5-4.5 USM; EF-S 17-85mm F4-5.6 IS USM, Canon EF 100-400 mm F4.5-5.6L IS USM, Canon Speedlite 580EX II, Canon G9