I have three systems only because I'm not very organized. LOL!
The first is a spiral bound notebook that I keep in my room. I mark each page by season and as I get ideas while I'm putting away laundry or trying on clothes, I note the outfit on the correct page. Here's an example of one of my entries:
orange and cream skirt/camel cardi/navy pleated bib tee/orange necklace/rose gold flats/gold stud earrings/orange-red crossbody bag
I usually pick my bottoms first when putting together an outfit because I have fewer bottoms than tops. So I always list the bottom or dress first and I can just scan down the left side of the paper for the bottom I want to style.
My second system uses the notepad feature on my Yahoo mail account. Basically it is the same as my spiral bound notebook but as I'm browsing fashion sites or on-line shops and an outfit comes to mind, it's right there for me to jot down. This system is a bit better than my spiral because I can look up "yellow cardigan" and it will show me all my outfits I've jotted down with that item.
My last system is the one I go to the most, my on-line photo album on Flickr. That's where I post all my outfit pics. I can see how things fit on me and be reminded of color combos that I liked. Since I make a habit of listing the individual items I'm wearing, I can do searches to see how I wore an item in the past.
So that's my crazy system. Hope that helps!