Tips for soft, chewy cookies:
Do not over mix the dough or use too much flour.
Bake cookies the minimum amount of time, even though the center may look slightly underbaked. Let cookies stand on baking sheet for one to three minutes to continue to bake, then remove to cooling rack.
Store soft cookies in an airtight container.
Do not store soft chewy cookies with crisp type cookies.
Use shiny aluminum cookie sheets,not dark colored ones.
Why Cookies Spread
Cookies may spread for a variety of reasons. So before baking an entire batch, bake a test cookie to give a good indication of dough condition. If it spreads more than desired, the dough may be too soft. Try refrigerating dough until well chilled (one to two hours). If the dough is still too soft, stir in 1 to 2-tablespoons of flour. Also, do not over-soften the butter before making the dough. Be sure to cool and clean cookie sheets between batches.
Another factor to take into consideration is the fat source used. If a lowfat or nonfat spread with 60 percent or less fat is used, cookies may spread.
For consistent and flavorful results, real butter is the answer for the appearance and taste bakers come to expect.
Over-baked and Cracked Cookies
Dry, Crumbly Cookie Dough
If your cookie dough is dry and crumbly, try adding 1 to 2 tablespoons of milk or cream. You may need to knead this liquid into dough rather than beating it in with a mixer.
Source and for more tips : http://www.fitnessandfreebies.com/food/cookies.html