I think the key to encouraging "shy" players to more actively participate is giving them plenty of opportunities to do so. When the party is formulating a plan and the shy player is just sitting back and observing, take a moment to get them involved. Have the party discuss how their character will contribute to the plan and what their thoughts and opinions are about it. Ask them if there is anything that the party hasn't thought of or possible flaws that they didn't consider.
Try to get each player engaging with the "shy" person (not all at once, but spaced out so as not to be overwhelming) through role play and by asking them for their input during the course of the session. This way, you'll help foster a better comfort level within the group and they may feel less intimidated about offering their active participation.
Also, make sure the actions and decisions of "shy" players have meaningful and exciting results. By showing them that they can have a great impact on the game and the story, you are showing them that you want them to be engaged, that they are important. And try to give the player an occasional encounter or plot where they are the focus. The other characters are of course there to help out, but make it clear that it is the "shy" player's character whose decisions are the most important.
HARRY DRESDEN — WIZARD
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