Pressure treated lumber (Within 2 weeks of purchase or wet)
Place the boards directly next to each other (edges touching). The moisture in the lumber will dry within a few weeks and leave a small gap between the boards.

Pressure treated lumber (Older than 2 weeks or dry), Cedar or Redwood
Use either a nail, a screw or a deck spacer tool to create a small gap (1/8"-1/4") between the boards.

Composite Decking
Use either a nail, a screw or a deck spacer tool to create a small gap (1/8"-1/4") between the boards. Check with the manufacturer before starting for spacing recommendations. Some products require specialized spacing gaps.