Playing and singing at the same time: Tips and advice?

I'm home alone, so I decided to practice playing and singing.

I found strange results:

I did two of the songs I know best, Gimme Gimme Shock Treatment and I Don't Care.

Those who know these songs, know that the first is high tempo, the second is pretty low.

I find it strange that I'm more all over the place with I Don't Care than Gimme Gimme Shock Treatment. I can't even make it halfway through I Don't Care without missing a note (or more than usual ) or focusing on one part more so than another. How can I balance what I focus on? I want to know what you guys do, like if you have a system or something that helps you.

Thanks

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1984 Squier Contemporary Stratocaster
1986 Peavey Envoy 110
1967 Kingston Acoustic

Watch this..

http://www.youtube.com/watch?v=atGBKuCJ-Jc

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I find that I just have to get to a point where I can play the guitar part without thinking about it. Don't forget you're allowed to simplify parts if its too difficult to play them while you sing.

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Rated "M" for meaty.