Lets look at the overview first.
You can plant anywhere within the square. Good hiding spot as CT is the lower right corner if you are crouched.
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_O1.JPG)
Planting on the top of boxes makes defusing an act of acrobatics and you can easily kill the CT with his focus on jumping.
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_P4.JPG)
To make it even harder for the CTs blow up the explosive boxes.
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_P7.JPG)
I didn't show this spot last night but you can actually plant in the corner. You need to jump there. If you do the bomb will actually drop down below.
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_P5.JPG)
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_P6.JPG)
And you could always use the little plantable area below. You need to stand precisely on the right spot to make it work.
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_P3.JPG)