been there, done that, worn the T-Shirt last march. At 36 months DS was assessed as at 24 months for expressive speech and 18-24 months for receptive speech (understanding) Since then, DS has improved a lot - he's still got a moderate language delay (so is a year behind for both) but uses sentences, can explain things that he's recently done, what he wants etc. I also find there are plateaus - that it's not linear progress at all, but stop -start -stop with progress.
What helped:-
1)agree with escape about the Hanen book It Takes Two to Talk. Lots of marvellous practical suggestions. You can get it a little cheaper (around £35) from a company called Winslow Publications
2)using very very simple language (if you don't suspect a delay with understanding, then may not be quite as necessary). so lots of "brush hair", "DS eat" etc - so trying to remember to use one word more in a phrase than DS currently could.
3)using photos/pictures to explain things/talk about places and family members
4)Using PECs (picture exchange system) to build up sentences.
5)Private SALT (couldnt' get NHS first appointment until DS was 3.8).
Ask NHS SALT if they have any courses/classes you and or DS could do in addition to his appointments.
BTW - SALT is in no way a "quick fix" - a lot of the work will be you and other family/carers working through the sort of games/excercises that NHS SALT recommend at the appointments. Also first appointment(s) may feel frustrating, as they can concentrate a lot on assessment at first.