One method is to just buy some books. The cost is low. And you might spend a lot of time to really learn a subject. However I am thinking more and more that hands on self learning is the best. You cannot replace being able to try things out yourself. Seeing what works and what fails is priceless.
Another technique is to attend a conference. This is similar to attending a short one week course on a topic. This usually costs a good deal of money. The costs get higher if the training is not local. The benefit is that you can learn much quicker. Unfortunately you will also lose the knowledge quicker if you do not put it to use right away.
Finally you can teach others. This can be at a local level such as presenting info to your team. Or you can step up and try to present at a conference. Either way you can learn more yourself by listening to feedback from the audience. The good news about presenting at conferences is that you usually get in for free.