To add to 2Wings points, I also included a letter of reference from my PPL instructor to say he thought I was upto the course, a couple of articles from Flight etc indicating pilot shortages( this was 5 years ago!!!!) and some letters from a few airlines outlining the wages i could be expecting to earn as an FO (some airlines wouldn't put the wages on paper but agreed to give a tel no. that the bank could ring to confirm wages.)
Good luck