Don't shop online
It's impossible to guess a shade using online guides. The colour of your screen could be completely different to someone else's, leaving huge room for error when approximating which shade to buy. Make a conscious effort to go into a store, spending as much time as you need to swatch and blend lots of different formulas onto your skin. Wherever possible, ask to take a small sample away with you to try at home under natural light; shops are full of artificial lighting that never show you what the true colour really is. The amount of money lost each year by buying the incorrect shade due to store lighting is utterly incredible.
Go for a premium brand
Unless you're really confident in your purchasing ability, head straight towards a manned makeup counter for one of the more premium brands. (Think Clinique, Bobbi Brown, YSL, Estee Lauder.) Not only will the beauty experts be able to provide advice on the type of foundation you should be investing in (you need to match the qualities with your skin type,) but they'll be able to take you through the broader range of shades on offer. On average, self selection cosmetics carry 4-8 shades of foundation for each range, whereas the more premium brands often carry more than double that. As no skin tone is ever the same, you're more likely to find a fantastic match if you have more variety to choose from. Not only that, but they'll be much more willing to give you a little bottle of product to try in your own time at home before making that all important purchase.
Don't swatch your neck
Your neck's skin is in a constant shadow, so can be a shade lighter or darker than your skin tone. You want to match your foundation to your chest, as this is what will catch the eye and provide a noticeable difference if the shade isn't quite right. Although it's always suggested you should go lighter if in doubt, I always find it more natural for paler complexions to go darker - using a too-pale foundation can wash away all the natural contours of your skin and leave you looking like a ghost.
Know your skin's undertone
Your skin's natural undertone will either be warm, cool or neutral. Your foundation should look natural and be used to either correct or enhance your skin tones, so it's important to know when you're shopping for a new shade.
You're warm if: You turn brown in the sun, have brown eyes, and the veins on your wrist are mainly green.
You're cool if: You turn pink in the sun, have blue or green eyes, and the veins on your wrist are mainly purple.
You're neutral if: You have a combination of all of these.
Change your shade every season
You're not one shade of skin for 12 months of the year. As you change your wardrobe, so you should change your foundation. As a general rule, go 1-2 shades darker in the summer months than in the winter. Alternatively, add a liquid bronzer to your foundation to warm and deepen the tones while keeping the match as perfect as possible. If you're worried about looking too uniform, add a touch of bronzer to wear the light naturally catches your face (nose, cupid's bow and apples of the cheeks.)
Application is key
You can have the best foundation in the world, but if you don't apply it correctly it may as well come from the bargain bin in your Nan's local chemist. For a flawless finish, use an angled foundation brush to apply to the skin in small movements, ensuring you use a little amount and build up coverage as you need it. Start from the centre of the face and blend outwards in sections, focusing on the nose and eye area to ensure the product is blended effectively. Fix your foundation with a very delicate dusting of powder over the t-zone, only where needed, to ensure it stays as long as possible.
Those are my top tips, but do you have any of your own? How do you ensure you get the perfect foundation for your skin tone and type?