Simple, “straight” cuts are best in most cases. Simple fades, where one image fades out as another fades in on top of it, are also a good choice. While fancy transitions are available on many video editing programs, page-turn effects, fancy wipes, and other complex cuts are distracting and make your video look less professional, not more.
Still photos of people mixed in with moving video shots can sometimes seem dead and lifeless. If your video program is capable of it, add a “Ken Burns effect” or "ease in/out" effect to the photo to make it more interesting and let you keep the photo on the screen longer without seeming boring. Both MovieMaker and iMovie have this capability. These effects are best when the movement is slow and either zooms in to or out from the most important part of the photo.
Generally, photos should be on screen no longer than 4-7 seconds each. Video clips can go longer, but anything over 10-12 seconds is probably a little too long. There are certainly exceptions to this guideline; it depends on the amount of movement you have to make and what the narration and music is doing.
Be sure to label everyone who speaks in your video apart from the narrator. The first clip of an interview should include a "lower-third" title telling their name and their title or relationship to the organization being featured in the video.
If you don't have the capability to do a lower-third title in your video editing program, at least make sure that the narration includes the information. This can be as easy as leading up to the interview clip with something like, "Stuart Conrad, one of the people that First UMC helped this Thanksgiving, was touched by the church's efforts." (cut to Stuart Conrad, who says, "It really means a lot to me that the people here did all this work to help people who can't make ends meet.")
Finally, be sure you save your video project often! Video programs process a lot of information at the same time, putting a strain on your computer's resources and increasing the chances of your computer freezing up or experiencing an error. Don't lose all your hard work! Save often!