By fabric: Separate delicate fabrics that require a gentle cycle from heavier items.
By colour: Separate whites from colours, and pastels from dark colours
By soil level: Soil can travel from one garment to another so wash heavily soiled garments separately
Lint: Check pockets and remove tissues before washing. Wash heavy lint shedders, especially blankets, chenille bedspreads, rugs and wollens separately. Also, clean out the lint filter in your washing machine regularly.