Courtesy of Mike Reagan, here's L.E. McCullough's presentation from the 2006 Northeast Whistle Gathering, called "Six Ways to Break Out of Being a Beginning Whistle Player – OR – How I Became L.E. McCullough (not that you would want this to happen to you or a loved one)." It's in Microsoft Word format, but you can get a free Word viewer from Microsoft if you don't own that program. If you've been playing for a few years you probably know all this stuff, but it's a great collection of important advice for folks just getting started.
Looking back on my first couple years of playing Irish music on the whistle, these are the things I now see made a big difference in my development. I sincerely hope they prove of some benefit to you.