One thing that really bogs down a trailer is adding TOO MUCH...too many transitions, too many effects... Simple is better by far. Here are the three transitions on moviemaker I use the most because they're hard-working and do the job everytime:
3. page curl (for multiple images in a row), and
4. shatter in where the denouement is revealed.
There are lots of other transitions, but the ones the are simple are the ones that seem to make the trailer more polished...
Next week I'll talk about effects : )