Recognize that people wrongly think that turning up the volume somehow creates instant understanding. Avoid this common mistake (however, do not speak too quietly).
Do not cover or hide your mouth because listeners will want to watch you as you pronounce your words. This helps them figure out what you are saying in many cases.
Do not use baby talk or incorrect English. This does not make you easier to understand. It will confuse your listener and may give the wrong impression about your own level of competence.
Avoid running words together ( Do-ya wanna eat-a-pizza?). One of the biggest challenges for listeners is knowing where one word ends and the next one begins. Give them a small pause between words if they seem to be struggling.
When possible, opt for simple words instead of ones that are complex. The more basic a word is, the better the chance is that it will be understood. ("Big" is a better choice than "enormous" for example. "Make" is a better choice than "manufacture.") However, with a Romance language speaker (i.e. Spanish, French, Italian, Portuguese, Romanian), these 'complex' words can be useful as they are rooted in Latin.
Avoid verb phrases that sound very similar to non-native English speaker. "Look out" sounds very close to "look for." Both are similar to "look out for." Many times you can use another word in these cases. (Example: look out = be careful, look for = search for, look out for = watch for).
As much as possible, avoid using filler and colloquialisms ('um...', 'like...','Yeah, totally.') as non-native speakers, especially ones of lower proficiency levels, may get hung up on these thinking the filler language is vocabulary that they don't possess. Colloquialisms are likely to be unknown as well, especially if they are not easy to find in the dictionary.
If asked to repeat something, first repeat it as you said it the first time. Then again. It could be that they simply didn't hear you. If your listener still doesn't understand, however, change a few key words in the sentence. It may be that they couldn't understand one or two of the words. Also repeat the whole sentence and not just the last couple of words. It's time consuming, but it helps prevent confusion.
Consider the fact that your dialect may not be what the other person has learned in school. For example, most non-Americans expect the second t in the word "twenty" to be pronounced.
Paraphrase. if you happen to know a similar word to the word you are searching for then use it. As your knowledge of the foreign language builds this becomes even easier.
Avoid using contractions or short forms. Use long forms. “Can’t” is one word you must use the long form with. It is difficult for a non-native speaker to understand the difference between “can” and “can’t” in a sentence. For example, “I can’t take you on Friday” and “I can take you on Friday”. Use the long form, “cannot”. “I cannot take you on Friday”.
Decrease the use of words that fill your sentences. The idea is to remove the “noise” from your speech. Imagine trying to listen to the radio with two young children in the same room. They are playing and screaming. What is the result? "Family of...car...on vacation...in Arizona." If your oral communication is filled with "um", "like", "you know", or other fillers, comprehension is more difficult. “Right” is a word that commonly fills conversations. I prefer to use “Yes, that is correct”. A non-native speaker may not understand “right” and confuse it with its opposite, “left”.
Be explicit: Say “Yes” or “No”. Do not say: “Uh-huh” or “Uh-uh”. Those words are not in grammar books!
Listen and try not to