Line up your stuff
Arrange what you’d like to pack from largest to smallest.

Roll soft items
Cylindrical objects fit into tight spaces better than squares or rectangles do.

Start packing
Working from the perimeter and moving inwards, place large items in first and work down to the smaller ones. Wedge soft items, like rolled T-shirts, in the remaining gaps. Instead of compartmentalising, you optimise space by using the entire suitcase.