J-Belle will no doubt be the authority on this, but this is how I remembered things.
Between wa and ga, WA emphasises that which comes AFTER (ie the focus is on the OBJECT), whilst GA emphasises that which comes BEFORE (emphasis on the SUBJECT).
For example;
Danieru wa compyuutaa wo tsukaimashita
Daniel used the computer.
danieru ga compyuutaa wo tsukaimashita
Daniel used the computer.
Conversely, the difference between ni and de is not in the emphasis but rather what kind is placed. Both refer to places, but in different ways. Ni indicates motion towards some[thing/where], whilst de signifies action within the location specified.
Maikeru wa daidokoro ni itta
Michael went to the kitchen.
Maikeru wa daidokoro de neta
Michael slept in the kitchen.
