The first thing we were given to practice in The Critics Group was a set of 4 open vowel(ish) sounds to find your natural physical voice so that you were able to get an unrestricted sound. I was surprised at the difference between the sound I had been using and the one I finally achieved. That was not necessarily the one I wanted to use, but it threw my field of options wide open.
Regular practice of the vowel sounds tended to keep my voice in good shape throughout the time I was singing.
You have to be careful how you use the exercises; overdo them and you can damage your voice.
You also have to choose where to practice them. Ewan and Peggy did them every day, and I was told by a hotelier once when I visited them when they were in Manchester, "oh yes, they're in the room with the strange noises".
There is a story in Luke Kelly's biography by Des Gerrihey, of Luke staying with a couple in Grimsby, who broke the bathroom door down when they heard oos, ees and ahhs coming from within - Luke doing his voice exercises.
When members of the C.G, were on tour, or when we were working intensively on something like 'The Festival of Fools', everybody carried a tin of Vocalzones (still available at Boots I believe). It said on the box "as used by Enrico Caruso".
Still got a few tins lying round the house; they taste like lumps of tar, but they work a treat.
Jim Carroll