Sirens are a good warm up (and cool down) for the vocal chords.
An oooh or even an ng sound can be used. Start at your lowest note, glide up to your highest then down to the lowest again. Never strain for volume. This exercise can remind you how much vocal range you actually have and yet don't necessarily use.
It is good to release tension wherever you can when singing and trills are used to relax the lips as well as warming up the voice.
Lip trills - sing a modified siren (not as low or as high) while blowing the air through your lips a bit like a horse.
Tongue trills - not everyone can do these. Essentially it is saying 'Brrrr' or 'Trrrr' as you do the trill and rolling your 'r's as you're singing the modified siren.
Vowels and consonants
When you sing, your body is a musical instrument.
Although breath and diaphragm/abdominal control is an important part of this, it is easy to forget that using your facial muscles and shaping your mouth also contribute to the sound you're making. They can help control it too!
Try singing the following: sing each line in a single pitch, then a third higher and a third higher again (equivalent to the 1, 3, 5 notes in a scale), then arpeggiate the sounds up and down.So you'll be singing each line four times.
While you do this, exaggerate your facial movements and notice the difference when you actually move your face and sing as opposed to only moving the lips.
ma me mi mo mu
la le li lo lu
da de di do du
ha he hi ho hu (notice how the diaphragm really needs to be engaged for this one!)