If you have a few hours left to prepare for your presentation, they are better spent rehearsing than fine tuning the design of your slides.
How to practice? I prefer to run through a presentation as if it were the real thing. No stopping, no stepping back, no rephrasing. Present it once start to finish, think what went well and what went wrong, and do it all over again.
In this way, you get used to the feeling of being on the spot, and that extra mental challenge will help you prepare better for your presentation.