Mostly I keep them in my head.
Like others have mentioned, a little black notebook for specific amounts of ingredients is good to have on you at all times.
And lastly, reading through this post, I just took a look at an application called "A Cook's Books" for mac.[EDIT]
I can't attach a link because I haven't posted at least 5 times, but here is their link: 3caam.com/products.html
I actually like the idea of these applications because you can get reference to nutritional values. Which is a humongous plus, personally.
I'm taking a look at the reference of nutritional values right now and there are over 200 nutritional categories and you can select, and even by amount, it's quite amazing and informative and very useful, in my opinion.
200.3 cups of regular beer (not light etc.) = 122,785.516 kcal of calories
and 17,132.863 mcg of "Folate DFE"! Whatever that stuff is, I don't know. But the information is there! I like it ...
I'm thinking about purchasing, it's not cheap though!