What are some exercises or tips to help singers improve their breath support?
Here are three exercises I use with students:
Lip-buzzing through a phrase of a song (i.e. one long lipbuzz - as you need to engage your support to lip-buzz). If you can't lip-buzz, then rolling an 'r' also have the same effect.
Slow breath in for three beats, then make a sizzling sound out for 10 beats (and then gradually extend this during practice to 15, 20 etc). For a greater challenge try a 'sh' sound (uses more air) or lipbuzz (needs greater focus).
Letting all the air out, and feeling what happens when the body takes a breath on its own (because it needs to) without your intervention. A teacher of mine called this "letting the breath take you!"
Two excellent exercises involving making fricative sounds and feeling what's going on with belly muscles are on Jenevora Williams' page at: http://www.stephengoss.net/Jenevora/documents/Vocal_exercises.pdf (Linked as I can't pass them off as my own!)
As an aside, the 'accent method' (a process which originated from speech therapy exercises) has been appropriated by various singing teachers to help with breath support. I haven't trained in this myself, but I see there is a course available from the City Lit in London. A summary, with some exercises, is described in 'Singing and Teaching Singing: A Holistic Approach to Classical Voice' (Janice L. Chapman).
One "game" is to get a piece of paper and place it against a wall. You then have to keep it up for as long as possible using only your breath. Without any support, you just can't keep it there.
If you make this competitive - against other singers, or just against the clock - it could be a way to encourage improving support.
There are 2 devices that can be used to help exercise one's diaphragm. In alphabetical order, Expandalung and PowerLung. The Expandalung is aimed more at athletes and comes with 1 resistance range. The PowerLung comes in several different models with different ranges aimed at different activities (sports for one series, music/singing for another). The Expandalung is less expensive, but the PowerLung has versions aimed at folks who are not peak athletes.
Disclaimer: I own 2 PowerLungs. I thought I'd only get the most manly/strong one, but it turns out to be too much resistance, so I quit using it and got the one I should have got the first time. I live in Denver and with the altitude, I can get out of breath going up a few flights of stairs - which I am trying to change.