First, of course you'd need a makeup bag or pencil case etc! Here's my favorite!
This bag is from Accessorize, it's a 'Simple Zip Cross Body Bag' but it's perfect as a makeup bag at just £15 I think it's totally worth it.
Oil Blotting Sheets - My skin is dry, usually. I always find that blotting sheets make your makeup look fresher after 5 - 6 hours of wear. And they're usually available at your local super market for just a couple of pounds.
Concealer - Always keep a tube of concealer in your makeup bag for quick touch ups or to conceal that spot that's waiting to strike.
Your favorite lipstick or lip gloss - I like to keep 2 lipsticks in my makeup bag - Tanarama and Hue, both lipsticks from MAC. They're both completely different, Tanarama is a gold shimmery lipstick that I like to wear when I want an added pop to my look, when I feel 'unique'. Hue is the perfect natural lip colour that I like to use when I'm doing my safe makeup!
Those are my main makeup must haves! However, it's also a good idea to keep some mascara and foundation in your makeup bag too. You never know what type of day you're going to have, so it's always good to have all makeup that you're wearing on you just in case you have a few tears or something.