flatness could be for a lot of reasons...
first, the fat content: Are you using butter or margarine or shortening? they each have different properties, and spread differently. You might also try refrigerating your dough before you bake it (expecially if you are using butter or margarine).
leavening might also be the culprit: is your leavening agent fresh? are you using enough of it?
I read the label on my Land-o-Lakes butter and had a 'duh' moment...
they recommend using pretty cold butter.
(take it out of the frig, cut it into chunks, and start beating at 15min)
they say that the chemistry changes once it warms (so don't use butter that was warmed and rechilled).
also they recommend not using a slick surface.
no non-stick, no silpat....
there is enough butter in the dough to keep it from sticking, so you want the resistance.
when I did this it really helped a lot.
it's funny, years ago, when I didn't know anything, I made really good chocolate chip cookies. then I "learned how bake" and while my NFSC cookies are great, my CC cookies stunk. I think that I finally got that figured out now 
btw, same goes for Oatmeal Rasin.
If you're using the Nestle Toll House Cookie Recipe, it pretty much su*ks anymore.
I remember making those cookies as a kid and even as a 20-something and they were ok. But in the past few years, they have ALWAYS come out flat, crunchy and just nasty. I think they changed the recipe or SOMETHING because there is definitely something different about them.
I refused to offer any chocolate chip cookies until I played around for about 6 months and developed my own recipe that bakes up great.
But the only reason I would cut the recipe off of the back of the bag is to use it as a bookmark!
Rant over .... shortening causes cookies to rise, butter causes them to spread. You can add more flour to a cookie recipe to make it a bit stiffer (but I added about a CUP more flour to the Nestle recipe and it STILL s*cked!)
Thank you so much for all this info. I will try adding the extra flour and using shortening instead of butter!!! Hope this works. Good to know someone else has problems with nestle toll house recipe too.. i use to bake em when I was younger too and dont recall this problem. Started to think it was me 
Not to toot my own horn 
but I've been told by many people that I make the best chocolate chip cookies they've ever had 
I'm not sure if it's true, but the cookies LOOK great 
All of the tips above have been great, and I have a few more. 1) if a recipe calls for 4 eggs, I use 2 eggs and 2 egg yolks. I learned from a Cook's Illustrated article that the whites can add too much liquid and make a cookie spread. 2) I use an ice cream scoop for each cookie or a plastic 1/3 cup measuring cup that I fill, then whack it upside down on the baking sheet. I press the dough down just a tad, then bake (my cookies are pretty big). 3) I use a combination of 2 parts shortening to 1 part butter. Hope some of these tips help!
Pebbles, it's your horn, darlin'.... you toot away!! 
1) if a recipe calls for 4 eggs, I use 2 eggs and 2 egg yolks. I learned from a Cook's Illustrated article that the whites can add too much liquid and make a cookie spread.
I'd never heard this but it's very logical. I may experiment with this idea!
Toot all you want!! Not enough bakers give themselves credit for great work. I work construction and take all my "practice" aka "messed up work" to them. They could care less how it really taste or looks, they are just so excited to get free goodies Thank you for the ideas, I really appreciate it
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