I have a bunch of tips I could share, I'll have to think of some first (I'll edit this)
1) XOR any register with itself to set it to zero. This is infinitely helpful.
2) Setting low numbers
Requiring most space:
MOV EAX, 12
Next least:
XOR EAX, EAX
ADD EAX, 12
Tiniest:
XOR EAX, EAX
MOV AL...