One thing you can do is pay attention to what the controller or FSS says. If you tune in and hear them ask someone a question (i.e. "Cessna 42F, please say type, position, and state request") then it would be wise to give it 30 seconds or so before keying the mic, since they're likely listening to 42F's reply even if you don't hear anything. Otherwise, just make your call and if they don't hear you, usually they'll ask "aircraft calling" to repeat. If you get no response, wait for a break and try again.

One thing you can do is pay attention to what the controller or FSS says. If you tune in and hear them ask someone a question (i.e. "Cessna 42F, please say type, position, and state request") then it would be wise to give it 30 seconds or so before keying the mic, since they're likely listening to 42F's reply even if you don't hear anything. Otherwise, just make your call and if they don't hear you, usually they'll ask "aircraft calling" to "say again". If you get no response, wait for a break and try again.