It's a bit of a tough one. While studying for the UMAT might seem like the logical thing to do in order to increase your mark, there's a lot of people that invest a lot of time and get nowhere and others that do nothing and do really well. Personally, I didn't want to invest a lot of time into something that could blow up in my face, especially when that time could be used to study and increase my marks in VCE, something which can be studied for. I did a a couple of practice papers in the lead up to the UMAT only so that I was familiar with the types of questions that can be asked and also learn a couple of strategies for solving the problems. I liked this approach so it's something you may wish to consider for yourself.
Should you take my advice, I strongly recommend buying Section Zero, which was, to me, pretty spot on for sections 1 and 3, and the explanations were also really thorough. It's definitely not worth Medentry or some other UMAT preparation course if you only intend to do this much. For section 2, I'd actually suggest you have a look at some of the passage questions in the multiple choice of the GAT. I found that these were more similar to the real UMAT questions than any other material I had seen. If you have a bit of a Google, you'll also find some free UMAT papers floating around online, which you might find helpful.