Much depends on the audience one is signalling to.

Join organizations like Mensa

To stupid or average people, this is a signal of intelligence. To other intelligent people, my impression is that Mensa membership mostly distinguishes the subset of "intelligent and pompous about it" from the larger set of "intelligent people".

Associate yourself with games and activities that are usually clustered with intelligence, e.g. chess, Go, etc.

Again this works as a signal to people who are at a remove from these activities, because the average player is smarter than the average human. People who themselves actually play, however, will have encountered many people who happen to be good at certain specific things that lend themselves to abstract strategy games, but are otherwise rather dim.

Speak eloquently, use non-standard cached thoughts where appropriate; be contrarian (but not too much)

Agree with this one. It's especially useful because it has the opposite sorting effect of the previous two. Other intelligent people will pick up on it as a sign of intelligence. Conspicuously unintelligent people will fail to get it.

Learn other languages--doing so not only makes you more employable, it can be a big status boost

This one seems like it might vary by geography. It's a lot less of a distinction for a European than an American. In the US, the status signal from "speaks English and Spanish" is different from the status signal from "speaks English and some language other than Spanish".