Following on from Rueben's list of one-percenters, here are my additions to that list of the little things that help your team. (Read Rueben's first.)
Moving your defender
When you see a teammate about to cut long, and you are standing near the back of the stack, a little movement in to the disc can draw your defender's attention so they can't help defend the teammate's deep cut.
Not getting in the way
If the thrower has engaged the dump, you need to leave the space around the dump and thrower for the dump to cut into.
Motivating the team
If you or a teammate do something good, let the team know. Get excited. Celebrate. Motivate. A huge layout goal is a very valuable spark that your team can use.
Distract and draw the defence by starting your cut before the disc gets checked in. The defence will be less likely to help defend the main cut, and the thrower can throw earlier to the main cut (or even you!). T-minus, because you cut at T-minus 5 seconds, for example.
See what the thrower, cutters and defence are doing, by constantly turning your head. Have no blind spots.
Share your observations
Let your teammates and leadership know about any tactically useful observations: "Fred has only thrown forehands this game", "Don is poaching a lot".