I love baking cookies and I don't know what sort of recipe you're using but I like to use the American style ones to get big, moist, chewy cookies. I'm not sure I should be linking to a recipe from this site, but if you search in Google for chewy cookie recipes, there should be plenty around!
If you'd rather just tweak the recipe you already have though, from what I've heard, adding an extra egg yolk, using more or all brown sugar (retains more moisture than white whilst cooking apparently) or maybe reducing the flour a little might help your cookies remain moist and come out chewy.
I also agree with cooking crazy that you might want to lower the temperature to cook the batter more slowly - think how slow cooked meat comes out all moist and tender! Say maybe knock about 10degC off the recommended temp. Another trick is to only cook them until they are *just* dry and remove them from the oven but allow them to cool on the baking tray as they will continue to cook and 'set' as the tray cools down.
Hope this helps!