All kids develop at a different pace; however, there are milestones for a reason and when your child is not meeting them you need to explore your options.
At 18 months, I expect a toddler to have 12 words/sounds (ie: moo for cow). Chase did not have this and thus I had her evaluated with a speech therapist and there were some simple things that we did to improve her expressive communication.
Toddlers really love anticipation... That's why the game of peek-a-boo is so enjoyable for them. How does that translate to improving their expressive language? The anticipation helps to draw out speech. Saying to them 1, 2...and pause to say the three to see if they will say it. If they don't, you may say 3 for them but at least you give them the opportunity. And end it with something fun like a tickle or uncovering a basket of dolls.
Another way is to say "Ready, Set...Go!" and pause before the go.
There is a lot of pausing involved when trying to get your child to speak, unfortunately when you have an older child it is harder because they like to communicate for them which makes your life simpler but doesn't solve the issue of speech delay.
By two, your child should have a minimum of 50 words, and be putting 2-3 words together (i.e., mama milk, dada up, etc.)