I'm a native English speaker who talks to many non-native speakers.

If I could only give one piece of advice - slow down and enunciate. People can be very forgiving of poor verbal skills as long as they can understand the sounds coming out of your mouth. But many people who are not confident in their speaking end up mumbling or speaking too quickly - this ends up just sounding like static. It is easier to understand something said incorrectly but clearly than to try to make sense of jumbled sounds.