Totally relax your belly muscles, then magine you are putting on a really tight pair of jeans and zipping them up. To do this you need to draw in your belly button and muscles below it towards the back of your spine. You don't need to breathe in while you are doing them.
If this doesn't work, imagine you are on the loo having a wee and the phone rings and you must answer it. The sensation of tightening your pelvic muscles is very similar.
Or imagine someone has attached a piece of string to your belly button from your spine and is pulling the string / belly button towards your spine.
Keep breathing throughout this.
Hope this helps.