Most pec machines (the kind you do flyes on) can have the pins readjusted, so you can do rear delt exercises on them instead of chest work. then you sit facing the seat, and move your arms from straight out in front of you to straight out to your sides. this is the basic motion that utilizes the rear delts. it's essentially the reverse of a flye movement that you'd do for your chest.
bent over lateral raises are good too. same principle as a lateral raise, just bend over at the waist so that your rear delts are targeted rather than your medial (side) delts. the deltoid muscle has three heads, one in the front, one at the side, one in th back. seated lateral raises would target more of your side delts. I find cables better than dumbbells for doing any kind of raise as the tension on the muscles is more even and not so dependant on gravity, as it is with dumbbells.
rear delts get worked during back movements as well. basically any movement where you're pulling weight from front to back with your arms.