The only tip I can think of is to perhaps keep them in a rubber-sealed cookie jar rather than a porcelain one where the moisture can leave the cookies and the jar. (And definitely don't keep them for days in a paper bag.)
The recipe that I used to use, at the back of Hershey's chocolate chip bags, would always remain quite chewy. I think that might be the butter content, which would essentially deep-fry it while baking. They wouldn't only be chewy, they would be slightly gummy, and I liked it that way but there's no accounting for taste.
If you want non-grease chewiness, then I might also suggest adapting a brownie recipe and just cooking them like cookies. Brownies tend to remain much softer because it's basically a cake rather than a cookie.
