Speak slowly, annunciate clearly, use only standard R/T, and be adaptive to the ATC environment you are in.
And by all means avoid the "Chuck Yeager" slang and loose pronunciation endemic among many crews....from the US in particular. I say this as a North American myself. I am frequently appalled and embarrassed at the R/T I hear from my fellow North Americans. Invariably, the poor Asian controller doesn't even respond, and I can picture him/her gobsmacked and confused as to what they just heard. This leads to repeat transmissions that just clog the ether.