Given the fact that love nikki
Dress Up Queen's main feature is collecting and mix-matching the thousands of outfits to be unlocked in the game, it is also the most crucial thing to be prepared for in the game. Meaning, you'll eventually have to keep track of hundreds of items later on in the game. To be anywhere ready to take on this task, you need a 3rd party app (Nikki Guide App for Android Users) to keep track of the stuff that you already have, and easily sort them out for you as you need them (during battles, you'll be required to meet a certain theme or motif -- this app filters the items you have as you need them). For non-Android users, you can also go to the Nikki's Info site, create an account, and keep track of your items there.(keeping track of all the hair-styles alone is a monstrous task in this game)
However, do take note that the app also happens to suggest items to wear in order to get an S-rating for a concluding level, which would somehow ruin the experience altogether. However, it is by far the easiest way to keep track of everything you've got in your inventory, unless you are keen to tabulating everything in a spreadsheet yourself.