When you are provided with numerous videos of varying lengths, and your final product can only be 30 seconds to 90 seconds, it's important to be selective and choose your clips carefully. Not only that, but you have to be able to tell a story. You choose what type of story you want to tell and with what type of material. Whether or not you are given a voiceover, will also determine how you arrange and sequence your video.
There's an art in keeping your audience engaged, and sometimes that includes adding visual effects or typography to break up the scenes or add cohesion. But the main strategy is to get your point across in a way that speaks to your audience in their own language. The key criteria to creating a successful video clip: timing, pacing, selectivity and most important of all, understanding your audience.