Keeping them fresh
To keep your potatoes in the best condition until you use them, keep them in a net bag in a cool, dark place. Put an apple in the bag to help stop them germinating, and change the apple once a week. Never put potatoes next to onions: it rots them!
You need to use potatoes with nice firm, waxy flesh. Cook them in their skins and peel them while they're still hot. Make sure you prepare the salad while the potatoes are still hot so they absorb the sauce better, but to stop them from absorbing too much of your sauce and seasoning, soak them in dry white wine after peeling and chopping, drain, then season and make the salad.
A few golden rules: use floury (bintje) potatoes and cook with the skin on. Peel and mash using a proper masher. Add butter, cream, salt and pepper, mix and serve.
Use large bintje or charlotte potatoes. Cut into chips, soak in cold water for 30 minutes, drain and dry fully using a tea towel. You can either stop to drain mid-way through frying or leave the chips until they're fried - it makes no difference to the quality.
Cook them in their skin the day before and keep them in the fridge overnight so that the starch goes 'stale' and you get a nice firm texture. Cut into slices and sprinkle with flour before you sauté them to make them nice and crispy.
They'll cook faster if you soak them in salty water for 20 minutes before you cook them.
Wash and chop your potatoes in half. Make a criss-cross over the exposed half, pour a teaspoonful of olive oil over each, add a pinch of salt and bake for 30 minutes at 180°C.