Flat sheets together, fitted sheets together, pillowcases (ironed) together.
Towels - grouped by type and colour, so the stacks are even lol I am totally OCD about my linen press (thanks Mum! xo lol) Bath towels together, hand towels together, bathmats together and flannels together.

If you press your folded flat sheets with an iron after you have folded then you will find they flatten quite a ways without having to iron the sheet itself