I keep it all in my head, more or less. Which can present a problem because, for example, at this time, I would easily forget what I had the previous spring or summer. I have brought home items only to discover I already have something very similar that I forgot I had. Then it goes back.
I do have all my outfit photos on my hard-drive too, which I'm finding is very very helpful. It reminds me of clothes I wore last year that may still be tucked away in storage. I would really love to have all my clothes out in view, but I am afraid that is never going to be possible since I have such a small closet. However I am getting closer and have made a lot of strides. I have less in storage now than ever. I did just put away my winter jackets (tweed, darker colors, heavier fabrics) and swapped back in my spring/summer jackets. Instead of rotating out 70% of my closet twice a year, I'm hoping to only have to swap in/out 20% or less.... right now I'm probably at about 40% which is better than 70%!!!
Sorry got off-track...
Otherwise, if I have a top that I want to find a skirt to go with, I will wear it to the store. I find this is very, very helpful. I can't tell you how many times, for example, I will end up buying a top just because it looked really cute with the necklace I happened to be wearing that day!
But it sounds like your dilemma is more about having recently acquired clothing that you have yet to wear enough to stick in your memory? If that was the case for me, I'd probably try the iphone method too (if I had an iphone), or go low-tech and print out a list to carry with me to jot my memory.
In my early YLF days, I did indeed take photos of individual items of clothing, then arranged them on a PPT to see what would go with what. I found that exercise very helpful. Not everything would work quite right once I tried them on together, but there was something about this method that opened up the creativity easier than actually pulling things out and trying them on. Don't ask me why!