Hey there! I'd be happy to share a couple things:
First and foremost is... you guessed it, practice. I know this is probably the response you'll get from anyone, but that's only because it's true. Keep making skins, and if you're satisfied with the result, upload them to Hive. Not all of them will be approved, but you'll receive plenty of feedback from reviewers and the moderator(s). Eventually, if you keep updating these skins and applying the comments of said people, they will get approved. You could also create a thread in the 'Texturing and Surfacing" forum for WIPs of your textures to receive constant feedback on every update (like you see some modellers do in the 'Modeling and Animation' forum). The only problem with this is that the texturing forums have been pretty inactive for a while now, and you might not receive as much feedback as you would uploading your skins.
Anyway, while practice is incredibly important, you can always improve you skill with specific types of textures through tutorials (and occasionally you might even learn a thing or two you'd be able to apply to all your textures (i.e. stuff about shading and highlights)). If you haven't already, I'd suggest you go through the 2d tutorials here on hive, and find some you like. Obviously, there are other (possibly better) tutorials out there on the internet, but these tutorials where made specifically for Warcraft III, so that's something to keep in mind.
One particular tutorial I was very fond of when I first got into skinning was Mr. Goblin's Tutorial. Now, this tutorial was made in Mr. Goblin's fairly early days, so some of the textures presented aren't incredible, but the reason I like this tutorial is because of the broad variety of textures it takes you through step by step. Also, the techniques used are something you'll be able to take with you when moving forward.
So, keep on keeping on, and I'll look forward to see more skins from you in the future!