Way #1: Mention something a lot of other people like, and say you like it too.
Way #2: Use the latest, hottest meme you know, and meme it like there's no tomorrow.
Way #3: No, there is no 3rd way.
And, there you have it!
P.S. I also heard rumors of another way to get thumbs up, but did not include it here for lack of solid evidence of it actually working. I believe it has something to do with posting interesting and/or helpful material, but I haven't seen such posts upvoted ever.