The simplest answer is that you oughtn't to do anything. Read a couple of articles by Brian Goetz I found:
this one, and
this one. The one thing you can do is to check for inefficient algorithms, eg
Using + or += in multiple lines for String concatenation.Using O(n^2) sorting algorithms rather than O(n log n).In general, as Goetz tells us, it is better to write simple code and leave the optimisation to the compiler, and not to try lots of micro-management. This applies to other languages too, see for example Miller and Quilici
the Joy of C where they have some figures about C optimisation.