The Very Basic Guide to Strategy:
If you're winning - keep it simple, the more rocks in play the more complicated things get and the better the chances of you giving up your lead.
If you're losing - opposite of above, get more rocks in play to make things more complicated and hope to get some points back.
If you have the hammer - try to force play to the outside so you have a bailout draw to the center for a point should things go wrong.
If you don't have the hammer - try to force play to the middle which will cut down the opponents area to make a bail out draw to prevent a steal.
Obviously this is just the general concept of strategy. Your decisions will be based on the team's strengths and weaknesses, as well as the opponents and what the ice may be giving you on any given day, but keep those general principles in mind and you're on the right track.