First thing, try putting some swing on your current strum pattern, mix it up.
If you Are strumming in A 1-2-1-2-1-2-1-2-1-2-1-2, try cutting in the next chord a little early.
ska is good training, gets you used to off-beat strumming.
Try And use triplets when strumming,
If the beat is a 1-2-3-4 try strumming A 1&2&3&4 down on the beat, up off the beat.
Im not really sure if im waffling or helping, let me know :P

are you putting the A as a capital for a reason? do you mean A chord or just 'a patten'

i don't entirely follow. I can change my strumming when I'm not singing but soon as I start singing it starts going back to what I described. erg!

just a pattern, I get 'shift' happy :D
Its all about practice and getting your hands to operate in a different rythmn to the one in your head.
Do you ever palm mute? That can help loosen up the strumming hand

er I've had a go at that on some neil young songs

I strum the song according to how it sounds on the album. I strum my originals as to how I want them to sound - changing patterns when there is a double bass coming in, or some different time signature.
Stevie Ray Vaughn is a good one to listen to if you want to break old habits. He has that swampy upbeat stroke that is like Hendrix, but clearer and more defined.

Sing your songs until you are so used to the words that you can sing without thinking about it and you can pay more attention to your strumming and licks. It's all about practise! By the time you hit the stage you should be sick of the song! Music is more work then you would think!
I've played for 35 years and my stage work is totally boring! Just the way it is!
Mark

http://www.gitarindonesia.com/