I'm sorry - that's so hard. My kiddo is almost 4 and we're finally making progress in the speech department - I understand about 1/2 of what he's trying to tell me (depending on the situation/setting) and other parents are picking up bits and pieces, too. We also taught signs and it was a big help (I recommend "Signing Time" if you are looking for a video - we have the set of volume 1-2-3 and it's fantastic).
Edit to add: I had to watch closely to learn "his" version of any number of signs - his "again" was a finger jabbed into his other palm, rather than all four fingers. His "help" was arms lifting up, like a fast touchdown, rather than a closed fist in the opposite palm. Once I figured THAT out, the window went wide open.
Early on, our speech therapist asked us to put together a little photo album of familiar items, to facilitate speech and also to assist in communication. So I took pictures of cereal, oatmeal, crackers, cheese, broccoli, salad, spoon, fork, milk, water, lemonade, car, train, truck, favorite videos/DVD player, radio, toothbrush, potty, Mommy's car, store, etc. etc. etc. etc you get the idea - about 60 of them. I found a small photo album and put all of those in there (trying to categorize as best as possible) and if I didn't understand something, I could say "show me" and he would.
20 months is a tough age anyway - lots of motor skills and so little language. It will get better, I promise. :)