Love pullovers, one of my favourite exercises ever!
I find it helps to focus mentally on the targeted muscle group, and also to contract those muscles as the weight reaches the furthest piont over the head, just before you bring it back over again. This then keeps the focus on those muscles throughout.
Also as you move your arm back over the head, keep this controlled. Don't let your arms flop back and don't think of this as a stretching move, i.e. how far back can you stretch your arms. Keep it more contained than that, make an arc with the arms and keep them in that arc the whole way over and back. You should then feel a stretch under the arm pit and along the shoulder blade as you reach back and this eases as the weight comes forwards again. If you are not feeling much of a stretch and no challenge at all in the entire exercise as you pull the weight back over the head, try a heavier weight.
What weight do you use for these? Cathe uses 15's in S&H I think, and that's too light for me. I use 25 or 30 pounds. And I feel it after 3 sets of 8 reps.
Clare