well i never had that prob so can't say much about it.
but i guess what u ask for shd come with a greater intuition and feel for music and rhythm.
u shd first learn only the rhythm without singing urself. u need to perfect the up and down strokes. try to play like the guitar on the song, trying to stroke acc to the rhythm it's playing, and try to make urself sound as much like it... cuz there is a distinct diff in the sounds of the up and down strokes and that can be made out from the song. once u understand the strumming pattern on the song play in the right rhythm with the beats and u shd get it.
i dunno if i can help more... it's not any technique here, it's more of a natural thing... u'll have to earn it with hardwork.
Many times I've lied, many times I've listened
Many times I've wondered how much there is to know...
Mellow is the man who knows what he's been missing
Many many men can't see the open road...
Many is a word that only leaves you guessing
Guessing 'bout a thing you really ought to know...