It's usually just a time+contribution thing. Just enjoy yourself, commenting helpful and witty things, and you slowly level up. I don't have a very high level considering how long I've been here, but my tendency is to lurk more than comment. If you develop helpful tools, or create something cool/useful to those here in the forum, that can be a solid contribution.
You can cheese the system and try to "game" more likes, but that's....well, what do you get out of it? Rather than aim for likes and higher levels, aim for those things you enjoy doing which naturally lead to XP. Just doing it for the XP means you're probably not enjoying the forum content, which is a shame. there's a lot of fun here.