As often as you can, you should consider trying out some new software from time to time, for the simple fact that new software can often be incredibly useful in creating new effects and so on. There are so many that you can choose from, so this is something that you are going to want to think about in particular, and it’s usually pretty easy and straightforward to find it too.
Remember that you can always - or usually - get a free trial on even the most expensive software, so that you can simply give it a go with no strings attached. After that you will often need to pay, but the more that you pay out, the more that you are going to get out of it, and the better your art will be on the whole. As such, it’s usually worth having new software whenever you can manage to afford it - and if you are selling your art, it will pay for itself eventually.