I have been able to get YouTube vid's to have a thumbnail appear in the SERP's on the first page for extremely high competitive search terms. Obviously this drives traffic.
The methods I used were:
Keyword in Title - in a natural (non-stuffed) manner
Keyword and link to the site in the Description
Keyword in the tags
Strong video views and embeds on other sites - I don't think I have it embedded anywhere on a high PR site.
Here is the video and keyword it ranks for so you can see what I did - Google search phrase - best lsat prep. The video thumbnail is in the 7th spot & receives several hundred views per month. Also #3 for the search term best lsat preparation on Google