1. count the beats of the song.
2. Note when the chord changes are in conjunction with the lyrics.
3. Practise your guitar part very very well ( This helps it to be 2nd nature to you and allows u to focus on singing with more passion too!)
4. Its also good to just keep repeating points at which u r struggling to sing and play at the same time.
5. Make sure you have a solid guitaring ability. This just really takes the stress off having to play and sing at the same time. (This I'm working on too!

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6. Its good to practise singing songs that have different guitaring rhythms just to get used to them.
7. Just for fun, try John Mayer songs..they are pretty hard to sing n play at the same time but sure get u learning to sing n play well.
8. Notice practice seems to be appearing again and again ?
Heh..just my personal opinion on what could be done. Hope its useful!
P.s. This advice is given under the presumption that you play guitar and sing at the same time.