Cognitive psychologists believe that our language construction is a result of a two-step word retrieval process.
Two Step Proceess
The first step is the search for a word's meaning, and the second step is the search for the actual sound of a word.
Different speech errors occur when you get stuck at different stages of this two-step retrieval process. When you incorrectly substitute one word for another, calling a cat a "dog" for instance, your brain has erred in the first step by choosing an incorrect word for the concept of "small domesticated furry pet."
Tip Of Your Tongue
When you have a word or a name on the tip of your tongue, you've gotten stuck in the middle of the retrieval process.
You've correctly retrieved the meaning of the word, but have been unable to retrieve the sound of the word. Because you've successfully completed one of these two steps, you can probably give the word's definition, but can't actually say the word.
How To Remember
Going through the alphabet one letter at a time might help you remember it.
When you come to the initial letter of the word, your memory might be jogged enough to help you complete the two-step process and remember the sound of the word you're looking for.
When you use an incorrect word that sounds similar to the word you mean to say, your brain has erred in the second step, when you were looking for the actual sound of the word.
So, for instance, if you said "Moment of Silence" instead of "Moment of Science" when you were talking about this show, your brain would have successfully completed the conceptual stage of the word-retrieval process but then would have made a slip during the second stage by choosing another similar sounding word that was more familiar than the word you needed.