Avoid poorly written villains.

In the OT:

Vader's a great villain because when we're first introduced to him, he waltzes into a room with a boarding party of stormtroopers unarmed & then proceeds to choke a guy interrogating him. Then later we see him choke out one of his own men with the force & use an interrogation droid on Leia. We're also told of how evil he is by Obi-Wan.

Tarkin's a great villain because he's calm, cool, and collected & Vader listens to him, and then he blows up Alderaan just to demonstrate the power of the Death Star.

Boba Fett's a great villain because the manner in which the bounty hunters are introduced as scum, & then Vader making a point to tell him that there would be no disintegrations.

Jabba we see loves to laugh at the thought of executing people. Classic criminally insane crime boss.

Palpatine's a great villain because of the way that admirals gulp at the thought of him personally overseeing them, the way Vader bows to him, & the way he just laughs at Luke & his optimism.

Compare to the Prequel Trilogy:

Maul is just a devil looking dude with a two sided lightsaber that kills a Jedi and promptly gets killed. He basically shows up for the sole purpose of having lightsaber battles.

Dooku just has a cool voice and cuts off Haydn Christensen's hand.

Grievous is a multi lightsaber wielding robot

The other minor villains are just really bad racial stereotypes

Sidious is OK, but we see him too much. The sequel trilogy needs to not show things until it's necessary to do so.

Give us villains from the start that are varied and unique. Fascistic military/government leaders, irreverent gangsters, and dark jedi.