After finishing a battle, rapidly press Esc several times before you see the 
exp dissapear. This will add up the exp points that you gained by killing the
monster every time you acces the inventoory and exit it (double Esc). 
It depends, of course, of your hands speed. This cheat also works when you are
between several monsters. Press [Esc] really fast, and youll see exp adding up.
Oh, i almost forgot, you can add exp points only untill you reach a new level, 
after it, you must kill a nother monster and repeat everything.