Like a lot of things involving guitar - take it slow to start with. Also, try learning realitively simple songs and move on from there, no sense in trying to run before you can walk, right?
Another thing that helped (or rather still is helping me) is the play along with records and sing. Personally, I found that by doing this things became a lot more natural to do.
Like others have mentioned, know your parts. Play them until they're second nature or even like a reflex. When you don't have to concentrate on what you're playing on guitar, it suddenly becomes a lot easier to sing at the same time.
Good luck, as always the key is practise, practise and more practise! (Pain, innit?!)
Don't worry too much about me, ignore me long enough and I'll go away.
