I just thourght Id drop in with a few things that have helped me.
But I a nowhere near a great singer.
I find when the day starts my voice is all out of whack, somtimes I can find a note to save my life. . . coming from somone who is rather well adjusted pitch wise. I used to hate singing becuase it made me feel so self concious even when Im the only one there. over the last few months Ive realised that a routine helps an awful lot. (beware my routine is quite the opposite of what you 'should' do).
If you need help with tuning, Ive found it helps to use an instrument you can sing with. for a strange reason I could never match my voice well with guitars. But when I started playing the piano, I could hold the pitch right on the money. What I do for practice is play a chord on the piano and pick a note and sing it, you know when your in tune because you can hear that you are (unless your tone deaf) I find this helps most because your learning to pick out the different notes of a chord, it trains you to follow a melody whilst listening to numerous different notes. It also helps to develop skills pick out harmonies etc.
Like I said . . . I cant seem to find my voice on a morning, because I smoke (although not much at all) I find (for me) that having a cigarette before I start kicks my voice into gear, but since I cut back and have being singin more I find it less important. Also don't jump into singing, listen to the song your about to sing a few times, while yourlistening to it jump around, dance a little it really helps put your head into giving it some. listen to some other music, again I don't know why but I find just listening to some music, playing the piano/guitar without singing shifts my ears into listening mode. Also although other pople say no, I drink some coffee I find the heat sooths my throat, and melts down any crap thats in there, the same with really hot food. I just think it helps clear out everything, gives my voice a bit of a thicker, warmer tone.
Be constructive. recently I've been singing a song, and comping it down (to find that it didnt need any comping

) But while listening to the takes I was working out the melody I had being singing and playing it on the piano so see how they matched up. referencing like that really helped me find out where I am going wrong, what notes im failing on and what I need to work at. . . evntually you can do I few takes and be happy with one! Like I said earlier I find it easy to sing with a piano, so I usually just play the block chords 1.1.1.1.2.2.2.2.3.3.3.3.4.4.4.4 It helps me pick the notes out better as well s keeping emotion, that I find is lost when sining to a melody.
Keep relaxed.
If you dont feel in a mood where you can sing, or you dont feel that you can feel the emotion of a particular song, then dont do it! it just wont happen, Like I said its good to jump and dance around to the song becuase it gets you in the mood. If your singing, don't push yourself. when your all warmed up everything flows fine, but as soon as you start to feel cirtain notes go sloppy and your voice start to weaken or feel fatigue. STOP! Just go out for a little walk get some fresh air, have a smoke (if thats what you do), grab a none alcoholic drink. just dont use your voice for an hour, then go back to it once you feel I little rested.
Hope this helps someone.
sorry for the essay