Sorry if someone else has said this stuff already but here's my input.
DS has speech delay, he is 22 months and doesn't really have any definite words yet. The thing that concerned our Health Visitor wasn't the lack of SPEECH - expressive language, but his lack of receptive language, ie he didn't recognise his name, or understand what you said if you asked 'where's teddy' or 'car for mummy'
I found all the rumours about extra bright kids who didn't talk until they were 4,5 or 6 to be less than helpful. i wanted to know what we could do because all the developmental stuff i'd read said by 18mo they should be starting both expressive and receptive speech.
We had a SALT assessment and they said you have to take it very slowly and build on their listening skills. So don't use complicated sentances and don't expand too much. for example if you are watching a Teletubbies video and a dog comes on, don;t say;
"look x doggy, you like doggies don't you, we saw a doggy today in the park."
Just say "doggy, doggy" and point at the screen.
Do listening games - like musical chairs/all fall down so they learn about listening for a change in sounds
Do symbolic sounds, so woof woof, ring ring
encourage imaginative play - thats another indicator of language delay apparantly. If the child has no imaginative play skills. DS wouldn't even push toy cars along or try and cuddle a dolly.