I am a crammer too but have become quite good at it, so good teachers don't believe me when I explain that I could have worked harder!
Tips: Get up early in the mornings, pick the key points to study.
Maths = learn formulas, look over past HSC qu's and try a couple if u have time, practice makes perfect and really without practicing maths daily u r going to struggle (sorry).
Look at a variety of qu's and make sure u think outside the square, especially in ext 1.
English: attempt to read over ur texts, at least a little bit. learn quotes and make sure u have understood ur texts.
Good luck!!!