Easiest way for me to remember, is to first figure out what UY I am traveling (not necessarily the same as the calendar year) and then I know I can use banked points from the UY prior to travel, UY of travel, and borrow points from the UY after travel.
So, if you are traveling in say June 2013, that is part of your 2012 UY, so you can use banked 2011 UY points, 2012 UY points, and borrowed 2013 points.
However, if you travel in December of 2013, that would be part of your 2013 UY and therefore, you can use banked 2012 UY points, 2013 UY points, and borrowed 2014 UY points.
We own a Dec UY and I think it can be the hardest because 11 of the 12 months of that UY happen in the next calendar year which can cause confusion.