Now, while it's nice if a model has her own kit, the following suggestions are for the 'tog who works with multiple faces. I have found I use these products on nearly every shoot:
The Makeup Kit Staples
-facial lotion, for both prepping skin and removing makeup
-clear lipgloss. Cheap Wet'n'Wild works great.
-4 brushes: one small stiff angled brush (brows and liner), one small springy flat round (concealer and detail), one round one the size of a fingertip (eye shadow), one large poofy brush (blush and powder)
-brow powder in dark brown. Again, go as inexpensive as possible. They all function the same.
-the most intense black shadow you can find
-shimmery white pigment
-black kohl eyeliner pencil
-blush in the deepest red available, peachy coral, and cool pink. It's better to get dark shades that can be applied with a light hand for the best bang-for-your-buck.
-a medium light shimmery peach-bronze for high-shine cheekbones (try the drugstore JesseGirl's loose pigment for this)
-highlighting or shimmer lotion/powder (I favor Benefit's "Highbeam" and MAC's strobecream)
-palette of cream concealer (MakeUpForEver's concealers are AMAZING, and are so highly pigmented only tiny dabs are needed. Well worth getting over the initial sticker shock for.)
-face powder to combat shine
Notice lipstick and eyeshadow is left off the above list. You'll collect quite a collection, never fear, but the trick is there is SUCH a selection available that it's best to plan the makeup for a shoot in advance, and only buy the colors that you need. Don't skimp on eyeshadows--buy quality brands like Smashbox and Urban decay--because the cheaper drugstore ones will have too much "filler" in them: they won't apply smoothly nor with any intensity. The only exceptions I've come across are the BenNye theatre makeup range, and the loose Jesse'sGirl drugstore pigments. Stay away from MAC eyeshadows too- they've gone really downhill in the last few years and contain more filler than pigment. The best thing to do is find a Sephora or other high-end makeup boutique to browse through to get an idea of what's out there. Eyeshadow can also double as lipstick, so look at buying the good stuff as an investment (apply lipbalm and then shadow color to lips, and coat with a bit of gloss). Lipstick is whole 'nother ball of wax. Dollar store lip color works just as well as high end labels.
What if you know you'll only ever use a color once? This is where I have to pimp out one of my favourite makeup sites: Aromaleigh.com. Find the eyeshadow color that tickles your fancy, and only buy a sample. They're normally under $2 a piece. That should be enough for one application. Aromaleigh also makes badass mineral foundation in an insane variety of shades; it's worth checking out their Glissade foundation and getting samples of a bunch of colors as a back-up foundation plan for if a model ever forgets to bring hers.
Any questions? Makeup is incredibly versatile and limited only by vision, so be sure to experiment and have fun!