Look around you-tube. theres a good few of them about. also I found that alternating between the highest and lowest notes I can make helps to warm my voice up
also
have you heard of the counting method?
1, 121, 12321, 1234321, 123454321, 1 - Repeat getting higher with each note successively, rising to the next octave on the last number 1 repeat until you reach your limit.
Also squeeze your cheeks so you make a 'brrbrbrbbrbrbrbbrb' sound (lol it's hard to describe I hope you get what I mean) and going through the scales like that. push to get higher notes.
hope that helps