I sing really well, and I play guitar really well. I just have problems doing both at the same time. I REALLY need to learn to do this, because I'm the singer and rhythm guitarist of my band.
Have no fear, the guitargadfly is here! I searched for open questions about guitar, and yours caught my eye.
Once again, I am amused at the ideas of some of your answerers. I chuckle softly 
It is fine to say "do it over and over until you get it right" – but I am afraid this is doomed to failure if you don't do it the CORRECT way.
The core problem people have with singing is RHYTHM. You have to learn to coordinate the rhythm of your hands with the rhythm of your voice. This is true when playing any instrument while singing.
The key to solving the problem is to keep it simple. I'll say that again:
KEEP
IT
SIMPLE
Most of us hear someone who is well practiced playing the guitar and singing, and blithely assume that if we just keep trying over and over, we will be able to do that as well. Wrong! You will fail. You already have right? It ain't working, right? So do this:
Play a simple song with simple chords- 2 or three MAX. Now use a down stroke strum on a simple count of 2 or 4 beats. Stop trying to do anything fancy. Stopppppppppppp it – no no no don't do that! Strum down four times in one measure. That's it. NOW try to sing the song while playing. NO upstrokes— NONE ! All down.
When you can do that and it is solid, then try adding an upstroke or strum on every beat. Progressively make the rhythm harder.
The CENTRAL point of failure in learning the guitar is that people try to skip steps and learn things too quickly.