My dog sheds a lot as well. I changed her food to Blue Buffalo which helped a bit. My vet also recommended I add flax seed oil and salmon oil to her food which really help. The omega 3s and 6s are really good for their hair and ours.
May I point out that the OP doesn't define "a lot"... my idea of a lot of hair and someone elses idea of a lot of hair may be different... For me, Shelties don't shed "a lot" - Labs do :)... but to a Poodle person, a Sheltie is going to shed like gangbusters...
I definitely agree - Omega 3 & 6 Fatty Acid supplements work wonders.
I hava a white shepard/great pyrneese cross. He sheds so bad and I have no idea what to do. I mean I have one of those furminators and i get garbage bags full of hair at a time. I will vaccum my seats and ten minutes later it will look like i haven't vaccumed in a month. I went to my local pet store and bought him some desheding suppliment to go in his food and have been using it for 3 months now and it hasn't got any better. I mean it is bad, even when he just leans against something or someone he leaves a wall of hair behind. You'd think he would go bald or something but he doesn't it just keeps comming off and it never stops. It happens all year around. It's not just limited to the summer like most dogs.