So I'm learning to play a new song (for many purposes), and I really enjoy it! I know how to play it perfectly when im not singing, and when i'm not playing I can sing it perfectly! But when i'm trying both at the same time, one of the other begins to slack.

I've only just learned this song about a few weeks ago (not a crazy guitarist and dont practice insanely often), so I need the best advice as to how to go about learning how to play and sing this song well!

Any tips of advanced guitarists/singers is welcome!!

BTW, the song is called "In Your Atmosphere" by John Mayer. It's somewhat of a challenge! not difficult though. The main problem I find is that it's easier for me to sing and play over a finger picking song as opposed to picking and strumming.
Ok, how much do you have to think about what you're doing when you're singing? If you have to think at all you definitely can't do both at once. You need to be able to do at least one without thinking about it so you can let that one start going and devote your actual thoughts to the one you need them for.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.

Quote by Master Foo
“A man who mistakes secrets for knowledge is like a man who, seeking light, hugs a candle so closely that he smothers it and burns his hand.”

It's true that yes, you should have the song down pretty well before singing to it, but once you get down singing and playing, that becomes less and less important as you go on. We recently lost out singer in my band and now me and my other guitarist have taken over the roll. It was really hard at first to split your brain like that, but just like with anything else:

Practice, practice, practice! Then take a break. Then practice some more.

We had only 2 weeks to learn to sing for a 45 minute set, so we would just run through the songs over and over again until we were satisfied. Now, 2 months later, I can pretty much sing to any part of any of our songs with not a whole lot of problems, and even improvising lyrics to new songs we have been writing.