Sugar has posted a great link defo worth a read.
My SN DD is in Yr4 and very behind in maths. Last year the SEN teacher helped me help DD by getting me used to maths language - one more, one less, add on, take away, plus, minus +,-, = ,equals, makes, sum of.
School uses numicon. Cubes that are colour coded and fit together.
DD has on recently been signed off speech therapy. DD and I used to sing counting songs on the way to school. This Old Man, One Two Buckle My Shoe, 10 Green Bottles, etc.
Something else we do , is play catch and count the number of sucessful throws and catches. As soon as one of you misses, start the count again. Good for hand/eye co-ordination too.