Have the butter as cold as possible. Handle the raw mixture as little as possible and chill pastry before rolling. Also, chill pastry after you have lined your tins as this relaxes the gluten, preventing tough pastry and shrinkage.
If you are not using a food processor, use your fingertips to combine the butter and flour.
When using a food processor, pulse the mix until bread crumbs are formed.
Add the minimum amount of chilled water - don’t add more water too soon and allow for differences between flours and their absorption rates.