When my boyfriend first moved into his apartment, I put together a list of things I wanted to keep there - that way I could stay over without having to bring a bag with me every time. You never realize how many things you use in a routine until you don't have those things with you (eeeek).
Although I wanted to have all of these things to make myself feel more comfortable, I also didn't want to fill up his apartment with a bunch of my crap either. Obviously every persons essential toiletries differ, but when it comes to keeping them at your boyfriends pad how you organize them is everything. I keep all of my makeup in a small bag which lives inside of a basket with all of my other essentials. Where the basket will stay is dependent on how much space and chill your boyfriend has, I keep mine on a shelf in his closet.
As for things outside of toiletries, boys will be boys and boys are M E S S Y! I make sure to have my own set of towels, and if you're as crazy as I am you'll even go out of your way to get him new sheets. I'm a girl that lives in a comfy robe and pajamas, so I make sure to have a great assortment of lounge clothes and of course a selection of underwear and bras.
As for clothing I keep a pair of workout clothes and sneakers, along with some basic tee's and sweaters, a couple pairs of jeans and a pair of casual sneakers. Just in case I stay over last minute and have somewhere to be in the morning.
Take it from a girl that's narrowed down, these are the essentials: