I take sentences that we have been doing in class and dictate them 3 letters at a time. So for the sentence (I have to go to school) I would dictate it in the following manner, pausing briefly for a slash (/). I H A / V E T / O G O / T O S / C H O / OL
They write down the letters - some will get in a muddle, one may hold their head in despair - but others will begin to see the sentence and start guessing what the completed sentence will be. A teenager once said he enjoyed it. I keep dictating until more than a quarter of the class request I stop.