Here's an abridged version of the latest answer I received:
"I know what I want to say.... but I just lose the 'word'.... So, How do I pull out... retrieve.... recall.... those very important 'words' when my mind has gone blank?"
This is a great example of why rehearsal is important. When you practice your presentation and have a clear idea of what you want to say and how you want to say it, you're less likely to forget your planned (not memorized) words.
Liza Minelli told this story to Ellen Degeneres last year, about how rehearsal helped her:
She remembered a team of doctors saying, "Liza, you will not walk again and you may not be able to talk but you're alive, and, for the attacks of brain encephalitis you had - you had 16 seizures - that's wonderful."
And her response: "I thought, 'Nah, nah, nah... There's got to be a way.' I thought, 'I don't know how to do anything else. What am I gonna do? Rehearse. I know how to rehearse.'"
She started "rehearsing" her speech and movement and had recovered 18 months later.
But practice and rehearsal is not the only way to ensure you remember your words.
This is the response I sent to the person with the question above:
"I find the best way to find the words when you need them is to keep reading and keep writing. Don't let your vocabulary drift to the back corners of your brain, but read, read, read and write, write, write. And then use your vocabulary in everyday conversation. Challenge yourself to use different words than the ones you always use.
Use your vocabulary muscles so that when you need to draw on your vocab, you have a deep well. And then, at least if you can't find the exact word in the moment, you'll have plenty of others to draw on!"
The truth is, we're all going to forget the "right" words from time to time. But by keeping your vocabulary fresh and robust by reading and writing on a regular basis, you'll find it easier to dig up the words you need when you need them.