If you have even slightly oily skin, blotting film is your best friend. I have oily skin and use blotting film 2-3 times a day to keep the shine away. I prefer these films to the regular papers (the film absorbs more oil). I bought mine at Sephora, but MAC has some (they are expensive though) or you can find them at Priceline (Australia) or CVS (US) from various drugstore brands.
If you're someone who occasionally wakes up to find that they've been attacked with a bright red blemish, applying a spot treatment throughout the day will help reduce the redness and get rid of that pesky pimple. I like Benefits 'Boo Boo Zap' because it's clear and you can apply it over make up (or under make up, if you're so inclined). It also has a cooling effect so if your blemish is painful it helps to soothe it.
Do you really want to be displaying that egg salad wrap you had for lunch for the rest of the afternoon? I don't think so. A compact mirror is essential to check for lipstick smudges, food-in-teeth or to just generally admire how beautiful you look today. You can find them super cheap at discount stores or drugstores.
A mini customised First Aid Kit
You don't want to be lugging around a whole bunch of crap you don't need so it's important to customise your first aid kit to include just the essentials. For me, it's cotton buds, band aids, alcohol swabs, a nail file, mini scissors and pain killers. All essentials that I find myself needing each week.
A Make Up bag with customised essentials
Everyone touches up their make up differently throughout the day. If you never reapply concealer, then don't carry one around with you. I always have a lipstick or 12 (which double as a cream blush), pressed powder, concealer and an eyebrow pencil.
A heavy duty hand cream
I work in an office building with lovely recycled air.. therefore my hands are always drying out. I love L'Occitane Heavy Duty Shea Butter Hand Cream, it's not cheap but it does last a long time (I've had my current tube for about 6 months and am just starting to reach the end of it). Look for a hand cream rich in shea butter and oils (I personally also avoid petroleum but it's up to you).
I stole a picture from google because my own is so beat up and almost empty it's utterly unattractive.
Water and a snack
Because bottled water is expensive so I don't want to be buying it every time I leave the house just because I'm thirsty. I keep a small bottle of water with me (so it's not too heavy) and I will also have a protein bar or two floating around my handbag. I personally love the Aussie Bodies Protein FX mini bars (especially the Chocolate/Orange and the Dark Chocolate/Cherry). They're only 100 calories each and have enough protein to fight away cravings.