Probably the most important factor:

• Shoot from a low angle.

If you can get low enough that you can see the sky under the person, it more clearly exaggerates that they are in the air.

Alternatively, shoot against a background with clear depth cues - A person jumping on a large, flat plane of a similar color provides no clue that they are actually in the air (Shadows can work for this too, if you have a bright light source, e.g. the sun etc...).

For example, in the picture in the OP, the girl is either jumping, or simply kneeling on the rocks. Since the image is 2D, there are no clear cues about the difference in depth between the girl and the rocks. If the angle of view was lower, so the mountains in the background were visible underneath her, it would be much more apparent she was jumping, and not kneeling.

Alternatively, place something behind them, that you can see.

Posing in the air is a different issue, and largely depends on what you are trying to do with the image.
Personally, I think it's fairly difficult to not look at least somewhat silly while jumping about, but that's more of a personal opinion than anything else.