Before Starting your PowerPoint:
1. Know and imagine your target audience so that your content will be prepared with their interest in mind.
2. Purpose of your presentation (wow my manager could be one). Usually the purpose is wrongly set up as "what should I present". Instead, we should always ask "what do I want to achieve" (I want customer to buy our product could be the 'purpose' of this presentation, also 'what I want to achieve'). This applies to every slide you make as well.
1. Use your mother tongue to prepare, this usually helps to get your logic and organization right.
2. Each slide has its purpose. Same methodology applies as point 2 above. Each slide should have its own purpose, and all together complete the purpose of your presentation.
Practicing & Rehearsal:
1. Write down your scripts and always pratice with the same scripts (of course at stage you could have your 'impromptu', but normally this happens only if you already have a script at spare). This will ensure you always choose best wordings for stage.
2. Voice recording your presentation (of course you could video record if you want!). Pay attention to contents, see if they are in logical, meaningful arrangement. Also note the intonation.
3. Last but not least, think if your presentation completes your purpose!
Presentation is a form of communication, or more specifically, communication of minds, and the art of communication is 'two-way', not 'one-way'. Keep this in mind and always create a 'two-way' presentation, although at a presentation you are normally the one doing most of the talking.