Common courtesy!

Give people plenty of room on a lake. Don't cut them off and don't travel too close behind. On some small lakes this can be difficult, apologize if you screw up.

On rivers, take notice of the direction people are moving. Don't fish water that they are moving into or until you determine their direction of working the moving water.

Don't move into a "hot" spot and push the other person out. You could always ask if they mind if you move into area.

I fish smaller rivers for trout and lakes with a float tube / pontoon boat, so I really can't address the line up on rivers when the salmon / steelhead are moving.

Offer tips, but only if asked.

Courtesy and consideration is the key.