1. Elevator Slides
Elevator Slides are a great way to work through your vocal registers and smooth out any breaks.
To do this exercise, you're going to make a noise similar to a long, slow siren. I recommend starting with an "ah" vowel and slowly ascend from low to high and back down again. Make sure you keep your throat relaxed and don't let your notes catch in your throat as you sing higher. This will happen if you don't move the resonance up into your head voice.
You can speed this exercise up as your voice gets warmer and switch the "ah" vowel to an "ee" and then "ooh," which work on a slightly different vocal placement each time.
2. The Lip Bubble
The Lip Bubble is a fantastic tool for not only warming your voice up quickly, but also for reducing vocal tension. It's very similar to blowing a raspberry like you may have done as a child. Here's how to get the lip bubble going:
Put your fingers into your cheeks near your lips on both sides of your mouth.
Relax your lips, jaw, and tongue.
Blow a raspberry without any sound until you can get that going consistently without breaking the flow of air.
Add an "ahhh" sound while you are doing the lip bubble.
Once you have that going nicely, you can use the Lip Bubble with any simple vocal exercise such as a major scale or arpeggio.
3. The Tongue Roll
Similarly to the Lip Bubble, the Tongue Roll is excellent for warming your voice up quickly and reducing vocal tension. The primary cause of vocal tension is your tongue, so it makes sense to practice rolling your Rs to keep your tongue loose and flexible. Practice rolling your Rs first and see if you can keep that consistent without breaking or spluttering for the length of one breath. Then add an "aah" sound to it.
While you're doing the tongue roll, do a simple vocal exercise like moving up and down three notes.
4. Vocal See-Saws
The other thing you want to include in your warm-up is an articulation exercise. Really get your mouth moving. Mumbling is a huge problem for many singers, and that is where Vocal See-Saws can help you.
Starting at the bottom of your vocal range, in one breath, sing up and back over a major scale or successive tones. Do this exercise with the words "doo bah," which gets your lips and jaw working.
5. Octave Jumps (for singers)
Your warm up needs to be about more than just vocalizing and relaxing. You want to also be working on your intonation and connecting to your breath, and Octave Jumps are a great way to do just that.
Keeping your notes staccato (short and detached), start from a low note in your vocal range and quickly switch between that note and the same note an octave above three times. Because you're keeping your notes short, your pitch accuracy may falter, so concentrate on hitting both notes straight on each time. Don't make the common mistake of falling flat on the top note or singing sharp on the bottom note. Then move this exercise up a note and repeat.
Ultimately, a warm-up should be as long or as short as it takes to get your voice to its peak performance level. I always test my voice by singing a range of songs that include my lowest notes right through to powerful belt notes. By doing that, you'll know if you need to spend some more time warming up or not. Just remember to stay hydrated and go easy at the beginning and build up to those belt notes. By the time you hit the stage, you'll be sounding great!