First, I recommend "Making the Terrible Two's Terrific" by John Rosemond and the Ted Tripp books on the "Heart".
If he's not the child who responds easily to a verbal reprimand (some children all it takes is a stern look and word, other's must be spanked), IMO, this is the age of an immediate, one good swat on the bottom for saying no. Then after the crying stops, following up, in a firm voice, with a sentence of- "his name" plus "is obedient and says Yes ma'am to mommy (or yes, sir daddy) when mommy tells you to (fill in the blank)." Then hug him, say I love you and tell him again what you want him to do. Then after he does what you told him to, then tell him thank you in an exerberant way (not forever, just at first; you don't want him getting overly used to exerburant thank yous). I would even tell him, though that next time, "G-d wants you to do what mommy says the first time", becuase you not only want to establish your authority but that your authority comes from G-d and isn't arbitrary according to feelings; that he is obeying G-d when he obeys you-the first time. Of course, this won't happen overnight, but just be consistant and he'll get the message quite clearly.
Or something like that, use your words as appropriate. Anyway, that's my opinion; you will get others on the topic of spanking, but there is a proper way to do it that is effective and it isn't abusive, won't teach him to hit, etc...