I started my UMAT preparation about 6 months before the actual UMAT - I find regular small doses of UMAT prep to be most effective.
Procastination won't really work here primarily because doing tooooo many UMAT questions in the last holidays will not help you for the UMAT - as it cannot be crammed for. Small doses of UMAT prep are good, throughout the year, they are even better.
So what shall be of the procastinators?
DW - There is no bigger procastinator than me right here

, infact I should actually be revising for my end of year med exams in a few weeks but ceebs - too lazy
So, If you can't help but procastinate - get external influence. Join a prep course or get a tutor, or work with some friends over the UMAT - I was "forced" to start my UMAT prep by having to attend a prep lecture in early january as I had paid for it. Hence - I was pretty much obliged to keep doing the UMAT through the year. Initially this was financial as I did not want to waste my $700 prep course - but as I did more and more I started to enjoy it
If prep isn't really your style - atleast do the ACER practice exams. Don't do them all very late like we all tend to do with VCAA exams for our VCE subjects - for the UMAT, do the exams periodically and do one as soon you you get them so that you can get an idea of what the UMAT actually feels like as early as possible.