Well, sign language isn't necessarily associated with EC, but it does make it easier for us.
We started doing baby sign language (ASL, in our case) for two reasons: (1) babies usually can sign earlier than they can talk, so they ended up with a larger vocabulary. Ruth currently has two words, maybe three: mama, dada, hi, but she has about 21 signs at this point. So, she can tell us if she wants bread or cheese or milk or water, and it makes things like mealtimes much easier. Plus, she can communicate about other things she thinks/hears/sees, like signing "dog" when she hears a dog bark, or "phone" when she hears the phone ring. (This was useful once when my cell phone was ringing, and she was down by my purse and could hear it, but I didn't.
) (2) She had basically started signing to us anyhow (a sort of "gimmie" sign, which is very similar to the ASL sign for "want"), so we thought it would be easier if we didn't have to guess at what her made-up signs meant.
For EC purposes, we figured we'd teach "toilet" and we also taught her to sign "change" for a diaper change. It means that we don't spend time watching to see what her personal cues are for needing to go, and works better for me since I'd rather have her be able to tell me that she needs to go than be guessing it. It's not for everyone.
The ASL sign for toilet is to make the T handsign (put your thumb between your pointer and middle finger in a fist), and then wiggle your fist back and forth a few times. When Ruth does it, she usually just smooshes her thumb up into her pointer finger or sometimes between them and waves either side to side or more commonly, back and forth. When people see it, they tend to think she has a "cute goodbye" wave--but of course, that's not what she's saying. (I just smile and start making my way toward a bathroom.) To teach it, I just signed "toilet" (and said "toilet" or "I'm going potty" or something like that) every time I went to the bathroom and let her watch and all (she was already pretty interested in what I was doing). And then when we got her potty, we also signed "toilet" while saying the word. And whenever she was obviously pooping, I'd sign "toilet" and she even started signing back in agreement.