What is the purpose of a preview clip? Easy - you should present your song in advance so that the greater audience of Eurovision is already a bit familiar with it before the big final.
It is also great if it can stand out from the rest and raise the interest in your entry. If people remember you better than the others, the is an advantage.
These days, when the previews are more for internet use than for televised presentations (in most countries anyway), a good clip is one that stands out, makes people remember you, invites to viewer to watch again and again. If you succeed at this, a lot more people will know your song in time for the final.
If they know your song, they are more likely to vote for it.
Svetlana Loboda from Ukraine knows a thing or two about how to get noticed. You can suck on fruit. You can fly a whip over your dancers. You can crawl around in melted chocolate. For instance.
Her preview clip for the 2009 Ukrainian entry is a big bad-taste party, a filmed orgy with too much of everything. Excessive images are thrown at you in a fast pace and before three minutes are over, you will be slightly out of breath.
A bit like a mental roller coaster. With music.
The clip isn't even as excessive as it looks - most of the images shown are bits and pieces from earlier Loboda video clips, very cleverly assembled to make them look all new.
Svetlana Loboda - Be My Valentine (Anti-Crisis Girl) (Ukraine preview 2009)
Too much of everything and I love it! In these times, when far too many people want to be cleancut, polite and politically correct, I can not get enough of the Loboda treatment.
Unfortunately, the whole package didn't translate as well as expected onto the live stage, and Svetlana was left in a 12th place - surpringly low and disappointing for Ukraine, a country that should never be discounted at the ESC.