Controlling Memory Usage

Use a sensible  $HistoryLength

By default ($HistoryLength=∞), and this causes Mathematica to  remember every 'In' cell and 'Out' cell during your entire session. This is  ridiculous because you would never make reference to an output from a very  long time ago as I do with  In[943]  in the next cell.

In[943]:=

To avoid eating up more and more memory you should put something like the
next line at the end of your (init.m) file.  That way $HistoryLenght will
always be set to 15 when you start Mathemmatica.

How to find your (Init.m) file.
Evaluating the next cell will return the directory where your (init.m)  file is located.

Save your notebook once in a while

Saving your notebook allows the Front End to return excess memory to the
Operating System.

Clear the Clipboard

If you Copy or Cut something that requires a lot of memory clear the
ClipBoard when you are done with it. An easy way to essentially clear the
clipboard is to select a small expression, a few words in a text cell, or
something else that is small, and select Copy under the Edit menu.

End with ; (a semi-colon) to avoid lengthy output

Ending with a semi-colon suppresses output. When an output is very long this
saves time and memory.

Created by Mathematica  (May 17, 2004)