I know that feeling, the plank is a killer. Sit ups are the best but I find that if they are not done properly you end up with back/neck ache. I have just bought (delivered this morning) an abs roller – used to have one years ago and they are brilliant. No way of not doing proper sit ups using this. Bought he Davina Ab Roller with Mat from Argos for £16 in sale – looking forward to trying it out in a bit. If you have an iphone get the abs workout app – it is free