There are a few items everyone should have in their makeup bag. With these you can create almost any look from natural daytime to a full on evening makeup.
Here is a list of my recommendations:
Powder foundation: I use one every day, it evens out your skin in a flash and sometimes you don't even need a concealer. My pick is Hello Flawless by http://www.benefitcosmetics.com/ which also has an SPF in it so you can top up your sun protection all day!
Black eyeliner pencil: This you can use to give subtle definition or go for an all out smoky eye. Useful to keep in your bag for touch ups too. My favorite is 24/7 Glide On Eye Pencil in Zero by http://www.urbandecay.com/ it stays on all day and is waterproof, I also like Oil Slick which is black with a tiny silver glitter in it - perfect for night.
Black powder eyeshadow: I always use black powder shadow over eyeliner to set it and you can use this to add drama too by smoking it outwards and lining under the eyes too. My choice is Black 4 by Makeup Forever available at http://www.sephora.com/
A palette of neutral shadows that you can create different looks with, going from extremely natural with light sheeny shades to dramatic evening with the more shimmery shades combined with the previous black pencils and shadows. I love 5 Coleurs in Earth Reflection by http://www.dior.com/
Eyebrow pencil: Your face isn't finished until you have defined your brows. It's amazing what a difference it makes. Choose a pencil in your shade from http://www.benefitcosmetics.com/ their Instant Brow Pencil comes with a spooly brush on the other end making it wasy to achieve perfect brows, even on the go.
Black mascara: I know everyone says you should wear brown mascara if you're blonde but I like the added definition and depth that black mascara gives to every coloring. My favorite is Deep Color Mascara in Black 01 by http://www.korres.com/ for US readers go to http://www.korresusa.com/ also available at http://www.sephora.com/ and http://www.studiobeautymix.com/
Powder blusher: This is the easiest to apply, especially for touch ups. I prefer a pinky peachy shade which suits most coloring, if you have very dark skin opt for a plummy shade. The one I use all the time is http://www.cargoscosmetics.com/ powder blush in Catalina.
Powder Bronzer: Great for any skin to add warmth and cheekbones if shaded underneath the apple! It's best to choose a matte formula that isn't too orange so it looks natural during the day and add shimmer with a highlighter at night. The best I've come across is http://www.jouercosmetics.com/ matte bronzer in Suntan.
Lipliner: A neutral shade can be used to define lips and add a base for gloss. I don't like just lining the outside of the lips, I always fill in too so that you don't end up with a ring around the edges when your gloss wears off! A great choice is Lipliner in Natural or Blush (for pinkier lips) by http://www.josiemarancosmetics.com/
Lipgloss: This is entirely your choice of texture and color. You can go for ultra glossy, plumping or a sheeny balm depending on your preference. Good picks are: Ultra glossy - Extreme Sheen by http://www.hourglasscosmetics.com/ , Plumping - Instant Plump Volumizing Lip Shine by http://www.get247beauty.com/ (also available at CVS), Balm: Sugar Lip Treatment in original (no color) or Rose by http://www.fresh.com/ which has an SPF 15, great for the summer months to stop lips drying out in the sun.
Eyedrops: After a long day my eyes get red and no matter how great your makeup is, this can definitely spoil a look! I keep a bottle of Naphcon A eyedrops http://www.alcon.com/ (available at all good drugstores) with me all the time, they not only relieve redness totally but also stop itching and irritation from allergies.
Last but not least I like to keep a makeup finishing spray in my bag to set my makeup but also refresh during the day. I cannot do with 10 Years Younger by http://www.skindinavia.com/ - it has come to my rescue many a time!