ME Meh Mah Mo Moo - - going up the scale and sliding back down on Moo. You can alternate with N instead of M for a different work-out. Keep it even! When you inhale, think of your first tone, in this case, Me. Start on top of the tone, that is, sound it a little higher than you think it should be as most people tend to sing flat. Think of the tones as being verticle as opposed to horizontal.

Also, Yo Ho Ho OO OO works well. Again going up and sliding back down. Sing these in a staccato rythym. Short bursts of exhaled breath.

Along with this, pick a song that you really like and that crosses into your higher and lower registers. Sing that before hand as then you'll be able to concentrate on your song when you perform.

Another suggestion, have someone listen to you while you practice. Your ear can fool you and accoustics change, but your friend can make helpful suggestions.

Good luck and let us know how it comes out.