Laura,

There are lots of possibilities. Try this.

First break the 18 teams into three groups of six and have every team play every other team in its group. This takes 5 weeks. A schedule for a group of 6 teams is:

Week 1: 1v6, 2v5, 3v4
Week 2: 2v6, 1v3, 4v5
Week 3: 3v6, 2v4, 1v5
Week 4: 4v6, 3v5, 1v2
Week 5: 5v6, 1v4, 2v3

Use this for each of the three groups. Now, break each of the three groups in half (and a and a b), so you have subgroups 1a, 1b, 2a, 2b, 3a, and 3b, each consisting of three teams. Make each team in 1a play each team in 2a, each team in 2b play each team in 3b, and each team in 3a play each team in 1b over the remaining three weeks. A schedule for teams A, B, C to play each of teams X, Y, Z is:

Week 6: AvX, BvY, CvZ
Week 7: BvX, CvY, AvZ
Week 8: CvX, AvY, BvZ

Use this for each of the three pairings of subgroups.

Hope this makes sense. Good luck and have fun.
Victoria