defrag, scandisk, erasing free space



every few weeks i like to erase the freespace on my HD. 1st i clean out all the junk, basically all of my temp folders. the next part is where i get confused, because i've heard several recomendations that contradict themselves. this part consists of 3 things.....defrag, scandisk, and erasing free space. could someone tell me what order i should do these in, and most importantly WHY? i really want to understand the reasoning behind this.
the "erasing pictures" thread had alotta helpful information. should i do it in this order?

run scandisk
erase free space
run defrag
erase free space
Because of the possibility of Disk errors especially on a Fat drive, you should ALWAYS do a scandisk first.

Then a free space.

Then a defrag.

and if you really want to be therow, because defrag sometimes leaves bits of copies of files its moved , do another free space.