Help with Tacking Up a Horse!?

I've been riding for years, and have been having jumping lessons for about a year now. Because I go to riding stables I've rarely tacked up, though I always un-tack after my lesson. I know it shouldn't be too hard, cos you just do everything the opposite way of what you do when un-tacking, but I'm having trouble putting the bridle on. I get as far as getting the nose band on, and putting the bit in the horses mouth, but when I go to put the horses ears through the head piece, the bit comes out. I've only tacked up a couple of times recently, and have had to have help every time, and it's getting a bit embarring. Can anyone help?