Cliparts are old, ugly and lame. I don’t know how else to put it.
So are starburst and fancy shape you may see in ads or flyers, they don’t work well for presentations.
The only advice I can give you on cliparts is simple: avoid them!
They really look from another age, used and reused to become a cliché, and they lower the graphical quality of your slides. I know they are easy to find and insert… and that is also a reason why the look so lame.
I never liked them but in the past they were a source of graphical representations of concepts. Nowadays you have no excuses.
Axe the cliparts, there are plenty of alternatives you can use. Choose great looking graphics (see my resources page) that will communicate better and make you look more refined and polished. Go for real pictures instead.
Photos are more vivid, our brain can connect more with them.
Does that mean use only real images and avoid drawings? Not at all. Drawings and icons can be effective in some cases as I already stated. But those are not old fashioned cliparts (or advertising like starburst)! Those are quality icons or drawings that look professional. If you don’t find what you are looking for it is not to difficult to make it.
Your presentation, you and your audience deserve much better than cliparts!