I've been playing for a little over two years now.
First recommendation: Carry a whistle with you and practice whenever you get a chance, for a little at a time or for longer stretches. If you have to wait for a bus or a class, or have a lunch break, or sitting at a stop light or waiting for the bank teller, you can be working on little bits of tunes, trying to tighten up and sharpen transitions and ornamentation.
Second recommendation: Learn a tune well before trying to play it fast. A lot of whistling seems to involve muscle memory, and the more times you execute a motion incorrectly, the more likely you are to continue the mistake. I've found that when I learn a tune slowly and deliberately, I can play them flawlessly at faster speeds far more easily, because every time I practiced, I did it the right way. Think of it like this: Every time you successfully finger a section of a tune, you take a step forward, making a little connection in your brain. Each successive successful attempt builds up that connection, and each failed attempt weakens it or forms another one that is wrong. Getting it right every time, even if you have to go slow, saves time in the long run, and the speed will come. If you're having a lot of difficulty with a section, run through it really slowly several times, doing it right, to fix it in your brain.
I suppose I should also mention that there are countless ways to play any given tune, so don't get too fixated on finding "The Way" to play the tune. Listen to multiple versions of the same tune and figure out the basic structure. You can start by imitating the person playing your favorite version and you can "personalize" the tune later when your skills have grown. If they do a really flouncy version of the tune, you might want to learn a more basic version first, just so you have a better chance at playing with others. The Foinn Seisiun CDs are pretty good for this:
http://www.amazon.com/s/ref=nb_sb_ss_fb ... nics%2C228
Have patience. I get frustrated with my playing until I realize that if I had started learning whistle in Band in 6th grade, I'd be in 8th grade now, and nobody really expects professional greatness from an 8th grader, and anyone who does probably wouldn't let their kid touch a whistle.
Think about this: In two years, with steady practice, you could be pretty good, amazing your friends, being tolerated at sessions, on the road to awesomeness. OR, you could be picking up the whistle again, wishing you had kept practicing, and still wallowing in beginner terribleness. (acknowledgement of a spectrum of possibilities between the two extremes)
I started playing back in 2002, the summer before my senior year. Had I continued, I'd probably be living in a mansion right now, surrounded by Type II whistle groupies, living off the royalties for my #1 hit album "Electric Aluminosity". But I didn't. So now, I'm only at the beginning of year three of my whistling journey, laid off and going back to school, surrounded by people who would kill me if I wasn't so good looking (and needed for baby watching).
So keep practicing.
