1. Remember: it's not your time, it's theirs
The harsh truth is that your audience will get bored fast. The only way to avoid the drift-off effect (and the sudden rise of people checking their smartphones while you talk) is to provide value for them - not showtime for you. This means one thing: stick to a single core concept and skim everything else.
2. Start strong, end with a bang - and make up a show in-between
Your goal is to make an impact: do it from the start! Share a fact, crack a joke - or go for both. Out of opening ideas? Ask a relevant question and have your public raise their hands. Action is always effective. And talking of action... don't forget to act: stand straight up, project your voice, speak slowly. Use silence instead of "um", "I mean" and the likes.
Repeat your key concepts shamelessly. Use a prop to stress your points: could be anything from a pen (when talking contracts) to a basket ball (when praising teamwork). Don't forget that a speech is about emotions. Those emotions are meant to be released at the end of it, and that's why your ending is key. Take your one and only concept, make it a great closing line - and build your whole speech towards that grand finale.
3. Always rehearse, whether you have 5 days or 5 seconds
Is your speech scheduled some time in the future? Good. Take notes, write a list or a draft, then start giving the speech to yourself again and again. Use your notes as a guidance (a mental guidance will be great) but don't read a full text! That'll look cold and detached. And don't speak in front of a mirror, for it's highly distracting. Is the speech a last-second proposal of your overexcited boss? Better. Conjure up an outline while the room is being prepared and everyone gets ready to listen - and boldly go!
You may not be able to give the ideal speech your first time around. You may leave something out. But that's what questions (and follow-up emails) are made for. Don't worry too much about not giving away enough information: a speech is all about how you make them feel.