I'm a singing teacher in my spare time and all of the above sounds good as but it's hard to tell what the problem could be without seeing you sing. All of the above are really good habits to get into and as much as it totally sucks, vocal warmups are essential. I actually do them twice during the day before a gig at night so you don't have to do them at the venue. Doing a light warm-up in the morning in the shower is really good (just humming and crap) and then doing another one mid arvo will work fine before a gig at night. Also the best advice that i ever received is to physically warm up before you sing. It makes a huge difference and engages the muscles you'll have to use while you sing. Running on the spot for 5 minutes, jumping, anything will help and will greatly improve your voice production. Also, pretend like you have something at the back of your throat like an apple or something (snigger). A big cause of tension in the voice is not opening the BACK of your mouth enough. You'll get the right idea if you yawn and stop at the climax and notice what your mouth feels like.
If you are ever in Wellington visiting or playing, hit me up at Medusa and I'll give you a free singing lesson or two. That's a saving of $60 p/h!!! That price is never to be repeated!!! hahahahaha
Ok I'm done.

