Pick the most distinctive part of the clip. A search by video samples five frames from wherever the clip starts and ends, so trimming to the scene with the clearest subject — a face, a logo, an unusual backdrop — raises the match rate noticeably.
Avoid black frames, transitions and pure-motion scenes. They carry almost no visual information, and a search by video that samples them mostly returns noise or nothing. Content with text on screen is the strongest signal of all, because indexed pages usually keep overlays intact.
And remember that the search by video sees the public web index only — pages behind logins, private accounts and freshly posted videos are invisible to any matcher, no matter how distinctive your frames are.