Use simple words that can be understood by the broadest audience. If your audience is already familiar with a term, use it–don't try to invent a new one. Be clear about what's going to happen if the user takes an action.
If the UI is overcomplicated, imagine what it must seem like to someone outside of your product team with less context. Listen to your customers and conduct research to find problems.
Edit messages down to minimal character counts when possible to reduce cognitive load. People often assume how a UI works without reading anything. Content should be parsable and to the point.
Can a two-page form be edited down to one? How many clicks does it take to complete a task? Can it be one click? Can you prefill anything?
Use design patterns that people have seen before and know how to interact with. Look for similar designs and language across the industry and within your own product.
Consider the emotional state of the customer and use an appropriate tone as a real human would.