I find that I am not very good at getting the players to be self motivated in the plot. Like in storytelling games where the players can have more control on the plot by deciding to go here or there. The world is open to them but they just want to sit in a circle waiting for something to fall in their laps.

The solution? Drop something in their laps.

Sometimes, it takes the scaffolding on a metaplot to get things going. Whatever town they're in? It's under-defended and goblins (or whatever) are attacking. Introduce a number of highly likable NPCs who recruit the party to help defend the town. Send whichever NPC the party seems to like the most off to Elswhereville while the party's hired to solve the goblin crisis. The investigation leads the party to Elsewhereville where they find out Likable mysteriously disappeared, the goblins have some sort of connection to Baron Bad Guy, and the sister from one of your players' backgrounds has been robbed.

Plot hooks are your friend. Even in a sandbox game, you still have to pitch the paths to your players.

Here's a Gnome Stew article on driving a sandbox game with minimal effort.

Interesting, the first four posts on the GM forum all come back to what I was preparing for my Campaign HQ article... plot hooks! They are indeed one of the keys to keeping a game going, be it sandbox or otherwise. The only real difference between a sandbox game and a railroad game is the degree to which players are encouraged to drive the plot on their own.