Drink tea with honey to warm up and lubricate your throat. Drinking warm tea will help to get your vocal folds primed, while honey will help to lubricate the vocal folds, making it less likely that your voice will crack.
Breathe in deeply to the diaphragm to help create a solid and persistent airflow. The voice will often crack because it isn't being supported with enough air. Proper air support often solves the problem of a breaking voice.
Sing the word "hmm" near the top range of your voice and slide down through all of the notes, creating a blurring of the notes. Descend five pitches for the best result, then start a half step higher and descend five pitches again.
Avoid singing in the top or bottom of your range. If a piece is generally too low or high for you, ask to sing another part. If it is a solo, you can change the starting pitch to a more comfortable pitch.
Sing major scales daily in quarter notes at about 60 beats per minute. Major scales will take you through the entire range of pitches and make you more comfortable singing.