I'm trying to start shooting people again.

What can I improve on?

I think the question of "What can I improve on" would depend on exactly what kind of shot of the person you're trying to accomplish, personal, artistic, emotional, glamour, etc...

But that aside, it's obvious you know how to properly expose and compose an image. As for shooting people, it always helps to have people that are in fact comfortable in front of the lens. The first image, the girl looks just a bit shy and maybe a little uncomfortable. If she had her chin up a tad higher, it might portray a little more confidence and relaxation in her.

For the 4th image of the girls face, it's just my little quirk, but I find this one odd... Did you use a wide angle lens? Perhaps using the rule of 3rds to place her eyes less centred would make it a little more flattering and a little less "in your face" to the viewer...

For the last image, always watch for hair elastics on models wrists... My all-time pet-peave for girls I have now! hahaha Even when I'm just out and about with friends I they have one around their wrists I get annoyed and make them take it off =P

And always remember to just keep trying different things! Practice will always improve your work.

There are also a few photoshop techniques that would really make the girl in the first picture eyes stand out, which i think would really make that photo pop...