Some tips for hitting the high notes:

1. Always practice at 'performance' level volume; this means that, whilst in performance, you are comfortable at your full range. It's surprising how many people 'throttle-back' while they're practising.
Whilst practising you need to find out what your full range really is - not just what you think it is. If your local folk club or organisation runs singers' workshops, take full advantage of them - they really can help.

2. Don't throw your head back when trying for the high notes - keep your chin low (counter-intuitive, I know, but it does work).

3. Think about where the breaths come in a song. If there's a particularly high note coming up contrive to find a breathing space before it so that you've got 'enough air' to hit the note. Proper breathing is of vital importance to a singer - although I've been singing for years I still need to work on my breathing - and will probably need to do so until I take my last breath!

Finally, I'll say it again, work on your singing with other singers - I have found, over the years, that proper constructive criticism is almost always helpful. All of the above tips have come from various singers' workshops that I have attended - thanks, guys!