Depends on what type of person you are john - but I would recommened starting to prepare about 6 months before the UMAT, along with all your other VCE subjects.
The UMAT cannot be crammed for - but that being said, it can't exactly be learnt either, so the best way to prepare is to continuously keep doing tiny amounts of UMAT every now and then - weekly/fortnightly basis to keeo you ever-ready for the real UMAT and constantly engaged with UMAT style questions.
Section 1 is difficult to "learn" - getting a good tutor to teach you methods and actually go through how to do questions rather than simply referring to the "correct answer" from the back of a booklet is better.
Section 2 is very difficult to learn - in many cases you are either good at it or not, but that being said, it won't hurt to do a few of these questions for revision - DO NOT overdo these questions as then your "section 2" instict will deviate away from using your natural empathy skills and you will start to treat S2 too much as a reading comprehension test- so do not over-prepare it
section 3 is very esay to imporve in - infact, do as many of these section 3 questions as you can - if you find out you are not as good at this section, then overload yourself with S3 questions and if you still do not improve gradually over time - consider taking a break and resume a couple of weeks later and or consider getting yourself a tutor to teach some of the general tips etc
Prep courses for the UMAT can be great and can be useless, they work for some and they don't work for the other - so it depends on who you are. I would recommend prep courses if solely you need something to boost your confidence for the actuall UMAT - otherwise if you are feeling confident and are not too happy about dishing out potentially hundreds of dollars and still not having someone to personally go thorugh the questions - then avoid prep courses.
For the record, I used a prep course which was very expensive but worth it and gave me immense confidence over the UMAT - access to a mountain load of S3 questions ensured that although initally I was plain shit at it - as time went on it became better and better and in the end it was my strongest section - I was always the same at section one irrespective of the prepr course - but did feel it helped a bit. I was shit at s2 before the prep course, and things were no different after it - infact I might have even done worse than I felt I would've - perhaps me doing way toooo many S2 questions made me loose the touch and feel you need to be able to handle them.
hope that helps