PowerPoint (and yes, Apple Keynote as well) offers way too many styling and customisation options.

1. Non-designers pick the wrong options: shadows, gradients, colours, fonts, and position things all over the grid without worrying about balance or layout
2. There are technical complications as well: copying and pasting slides across creates a mess of different templates, with different defaults. And even if you want to change something because it looks bad, few people know how to do it (straighten out column sizes, fixing that hanging bullet).

The objective of my presentation design software SlideMagic is to free you of all this stuff. If you have to work in PowerPoint, my advice is: make things look like SlideMagic slides! The easiest way is to work in SlideMagic, then convert to PowerPoint. Second best alternative: stay in PowerPoint.

Here are some of the steps I go through when I am faced with the challenge of cleaning up 100 slides of PowerPoint in a very short time:

• Copy the file, delete all but 2 slides, open the slide master, and delete all but 2 slides in it, so you are only left with a title page, and a regular page.
• Fix the slide master
• Create horizontal and vertical drawing guides in the slide master
• Do a brutal font replace across all slides to get rid of any legacy fonts
• Set the colour scheme, save and apply to all slides
• Insert a blank text slide, and create a shape and and a text box. Fix fonts, alignment, line spacing, padding, colours, right click them and set them as default shapes.
• Open the original file and copy all the slides, paste them in the small file you just created
• Select all slides (except the title slides) and change its format to slide 2 in the slide master
• Open the slide master and delete all master designs you don't need (i.e., you are again left with 2 slide masters)
• Do a global font replace again

So far, the presentation set up. Now follows the adjustment of each slide:

• Fix colours, kill: gradients, drop shows, glows, bevels, underlines
• Make sure everything fits in the slide frame, your drawing guides
• Cut text dramatically, take out duplications, indirect verbs, words like "in order to", cut, cut, cut
• Change vertical layouts into more horizontal ones: instead of "sub title, bullet bullet bullet", create a category for subtitle on the left, and put the (chopped) bullets to the right of it. Like the SlideMagic tables

Often it is just faster to re-create the slide from scratch rather than trying to fix it. And in SlideMagic, you will be even faster. You can't put objects in the wrong place, and all slides use the same basic "slide master". Good luck!

Image from WikiPedia