In addition, if you open the GUI (the graphical part of the program), you should always close it by hitting the 'X' so all memory that the GUI was using is released. Minimizing will not do this.
Keeping the client GUI window open, can result in the usage of large amounts of RAM, upwards of 60mb. To maximize the performance you should set all the graphics to low using the slider
... and turn off all the movement by moving the little joystick to the middle.
Credit to slash for this info and graphic.