Skipy;
I'm a bit disturbed about the advice that you should just get up and sing and not worry too much of how it comes out (that's how I read it anyway).
Only sing when you are ready to, and when you have done enough work on the song for it not to fall apart in your mouth.
Two reasons:
1. If a song is worth singing, it's worth putting the work in beforehand.
2. If you make a hames of it first time round, it will be twice as difficult for you to get up the second time round.
MacColl summed it up perfectly for me in an interview we did with him in the early 80s.

"Now you might say that working and training to develop your voice to sing Nine Maidens A-milking Did Go or Lord Randall is calculated to destroy your original joy in singing, at least that's the argument that's put to me from time to time, or has been put to me from time to time by singers who should know better.
The better you can do a thing the more you enjoy it. Anybody who's ever tried to sing and got up in front of an audience and made a bloody mess of it knows that you're not enjoying it when you're making a balls of it, but you are enjoying it when it's working, when all the things you want to happen are happening. And that can happen without training, sure it can, but it's hit or miss. If you're training it can happen more, that's the difference. It can't happen every time, not with anybody, although your training can stand you in good stead, it's something to fall back on, a technique, you know. It's something that will at least make sure that you're not absolutely diabolical
The objective, really for the singer is to create a situation where when he starts to sing he's no longer worried about technique, he's done all that, and he can give the whole of his or her attention to the song itself she can give her or he can give his whole attention to the sheer act of enjoying the song."
Says it all for me.
Jim Carroll