Use anything and everything you can, from books to video tutorials to interviews. I would caution about youtube videos though because the vast majority are full of really bad advice. I would stick to tutorials by industry veterans. Unfortunately, many of these you will have to pay for, but if you are pursuing a professional career it's a small investment. Most of all practice practice practice and always compare your stuff to the stuff on the charts and be really nit-picky about the quality of your stuff. I would also recommend finding a group of peers locally that you can always turn to for advice and feedback. Preferably you could find someone much more talented than you locally you can forge a relationship with.
I will say that I've been making records as my full-time job for about 18 years now and mixing is my primary income. I've had some minor hit records and I still spend time searching out info to improve my mixing skills - talking to colleagues, reading interviews with mix engineers, etc. At my level it becomes harder and harder to find info that I don't already know, but there's techniques people use that I've never used before so it's worth it.
I will also just add as an afterthought, nothing pushes you to improve like working for a client. You become significantly more picky with your work when you know it is going to be evaluated by a client who is paying you money. You probably are not in any position to charge significant money, but even just a courtesy $5 per mix is enough to turn the screws on you just a little bit. Find some home recordists that suck at mixing and offer to mix their song for $5. It will also give you good practice at meeting deadlines and just acting more professional in general.