I adapted these lists from handbooks for various university conversation partner programs, especially William and Mary’s.
Repeat and rephrase
If your conversation partners don’t look like they understand you, repeat what you just said. If they still seem confused, try to find a way to rephrase what you just said using different words and/or simpler grammar. Talking slower may help. Talking louder does not.
Focus on key words
Even the most advanced nonnative speaker does not understand 100% of what is being said. Instead, they hear key words. Knowing that, you can emphasize the most important information, slowing down for the main ideas.
Note important information
If something is important, note it on the wiki. You can edit it later.
Resist the urge to correct everything that your conversation partners say incorrectly. Consider the idea of comprehension versus perfection. Can you understand what they is trying to say? Let them speak, and when they are finished, you may gently remind them of a few of their mistakes. Try not to interrupt them while they are putting their thoughts together.
Abbreviations can be confusing to nonnative speakers. This does not mean that you should not use them; just understand that a nonnative speaker might not understand what they mean.
Limit slang and be ready to explain
Likewise, slang expressions can be very confusing to nonnative speakers, although they love to learn them. Your conversation partners will pick up slang expressions from you that you are not even conscious of using. When you think about it, there is a vast amount of slang being used. Do not overwhelm your conversation partners with everything you have ever heard. Try to stay focused on what they are most likely to hear.
Have a translator handy if possible
Though certainly not a requirement, it is helpful if one or both of you have access to an electronic translator. I recommend Google Translate.