Set your goal
To help the writing flow, play "join". Provide two short sentences that have to be combined, eg, "The cart stopped. Gandalf climbed down". Joined together, these could become, "As the cart stopped, Gandalf climbed down".
Provide a bank of connectives, such as: after, although, as, because, before, but, immediately, if, or, so, that, to, until, what, when, where, while, who.
Practise extending sentences by providing a simple sentence (Boris loved kippers). Let the children extend it by selecting a connective and adding on an idea (Boris loved kippers until he ate a rotten one). Practise until "joining" becomes automatic. Then make sure it happens in the children's writing.
Forget grammar exercises. They usually exercise nothing. Ted Hughes believed that the "conscious manipulation of syntax deepens engagement and releases invention". He was right - for it's hard to invent, if you can't write sentences that flow
Pie Corbett is a literacy consultant