I've always been a decent public speaker - certainly better than the average JW - so I rarely practiced for short talks. I would spend about 45 minutes, usually on the night before or the afternoon of the talk, preparing the information. Then I'd just stand up and give it. Because I was actually a little creative, adding a story as an illustration or something like that, I received a lot of praise from the congregation members.
One technique I used as a JW that set me apart (though some COs and DOs used it too) was to address the audience in the second person. The usual technique in the JWs is to speak in the first person (e.g., "Are we spending enough time in Bible study?"). I have found that doing so actually makes the statement less forceful. The speaker is presumably speaking because he knows something that the audience does not, or because he seeks to offer advice to the audience that they need. There's nothing wrong with addressing the audience as "you," and doing so makes the impact on the hearer more direct and personal. I don't think anyone actually noticed that I was doing that, but they certainly liked my talks.