Give it some thought. When you are carrying on a conversation, particularly with someone who is not a family member or close friend, what is there that you might like to change in order to communicate more effectively? Although it may be, this isn't necessarily related to vocabulary or your ability to enunciate. For example, I think my biggest obstacle is that too often, I find myself thinking about what I am going to say in response while the other person is still talking. This might be a character flaw, but I have to remind myself not to do that. When someone does that to you, you'll probably notice that they're just waiting for a pause in the conversation to get their own stuff in. I don't do that as often now as when I didn't even realize that I was doing it, but I still find myself doing that sometimes. Perhaps related, when I was a kid, I believed strongly that I could convince anyone of anything if only I had the right words.
I don't think fast enough to carry on a casual conversation like most people can. Not enough practice maybe, or maybe I'm afraid of saying something stupid on the spur of the moment. So, I usually just listen most of the time.
Probably that I don't have a "poker face" and when I'm annoyed by something, it shows. Also, since I retired I seem to use more "colorful" language without a filter. Somehow an expletive just gets the job done.
I believe the biggest obstacle to verbal communication is what words mean. The perfect language would be one where each word always has the same meaning for both speaker and listener. The language would be a one-to-one code. This way, there would never be a misunderstanding. But one-to-one language is not what we have. Our languages are one-to-many codes. Any word I use may have many definitions. For instance, if I say "John's life was a great success", one person might think, "John must made a lot of money". Another might say, "John must have been famous". Another might think, "John must have been a good man". Bottom line: To communicate more effectively, I suggest using clarity, brevity, and simplicity. Say as much as necessary to be clear, use no more words than necessary, and keep it as simple as possible.
First, unless I am the speaker and speaking to a group of people, I am normally not a talker. I’d make a lousy Jehovah’s Witness, Mormon or a Seventh Day Adventist who busy themselves with knocking on doors because I really don’t like to interact with someone I do not know. Secondly, to piggy-back on what @John Houlihan offered, although I am not trying to over-indulge the use of a larger words, there are times when I get a befuddled look from someone and I have to rephrase what I’m saying in order for that person to understand. It’s complicated but I do look for any indication of what level of grammar people are used to conversing with so I can speak to that level and be a little more comfortable whilst having to talk.
Yes, know your audience. Unless you are talking to an academic audience, use shorter sentences and shorter words. Like Herman Melville said, "The man of true science uses but few hard words, and then only when none other will do. Whereas the smatterer in science thinks that by mouthing hard words, he understands hard things". The Bible is a great example of the foregoing. It is written at the 7th-grade reading level, and yet it discusses some of the most profound ideas ever. Likewise, most successful novels written today are at 7th grade reading level.
I don't seem to have much of a problem while talking face to face. My bug-a-boo is talking on the phone. Someone will say or ask something, and if I take a second or two to think about how to respond, I get "are you still there?". I like to think before I speak but it seems on the phone any delay in speaking isn't acceptable. So when I get the "are you still there", I say yes, I was just thinking. Then the other person just goes on and on, and I never do respond to the first comment. The whole thing becomes a convoluted mess. I wish I could use e-mail all the time instead of the phone. I like having time to construct a coherent sentence.