Some great advice there from Silberfuchs.
I would add the following:
1. Keep it simple and stick to standard ICAO phraseology, especially when talking to controllers whose first language isn't English and/or when talking on HF.
2. When speaking on HF, talk slowly, 'deliberately' and a bit louder than you normally would. HF is often crap and the radio operators have as much trouble hearing us as we do hearing them through all the background noise and chatter.