Planning out your transmission works best.
I talk on the radio professionally (I'd say I fly professionally, but we ought not get paid for doing this, so it's a professional talker. The flying is free). Even so, I find that for some transmissions such as position reports, I often write everything down and read it off.
Otherwise, I usually compose what I have to say in my head, before pressing the transmit button.
In the event of emergency radio traffic, concentrate on getting the message out. Remember that plain English works just fine.