For most people, really nailing the lats is a hard task to accomplish. Except for those lifters who are already lat dominant, chin-ups, pull-ups, and pulldowns become arm exercises more than lat exercises. Likewise, unless you're lat dominant, rowing movements will hit mostly the mid-back, not the lats.
Use a shoulder extension movement to train the lats. In a shoulder extension, the biceps and mid-back aren't involved so they can't be used to compensate. A shoulder extension movement refers to bringing the arm from up to down–toward the rear of the body. The exercise that uses that movement pattern is called the straight-arm pulldown.
The Straight-Arm Pulldown
While pullovers also use that function, the cable straight-arm pulldown is superior to maximally target the lats because the tension is more constant throughout the range of motion, whereas dumbbells or a barbell only load half of the movement.
The straight-arm pulldown also involves the teres major–a good thing since it completes the look of the lats and increases the width of your back–as well as the posterior deltoid. These two muscles aren't strong enough to override the lats though, so the lats still take the brunt of the work.
Grip and Torso Angle
Shoulder extension is stronger and more effective when the shoulders are internally rotated, like when you use a palms-down grip on the bar. So keep your grip style the same (pronated), but vary the width–narrower or wider.
Also, change your torso angle for variation. A more upright torso will put more stress on the lower portion of the lats. Lean forward a bit and you'll feel it more in the upper portion and teres major. Try both versions.
Here's the key thing:
When doing an isolation exercise to target a specific muscle group, experiment to see in which position you feel the most tension in the muscle you want to stimulate. Remember, the goal is not to perform an exercise from point A to point B; it's to use the exercise to load the muscle you want. So the most important thing will always be feeling the proper muscle doing the work.
The Advanced Version
Once you've mastered the basic technique, you can try the more advanced version which combines both positions. Initiate the pulldown with the torso leaning forward. As you reach the mid-range point, slowly raise your torso to finish upright. If you nail this, you'll feel the strongest possible tension in your lats.
To get this movement down, use a slightly slower speed of movement than usual and focus on keeping the tension on the lats. Try to keep the speed constant during the whole rep. It should take you about two seconds to execute the concentric portion of the movement, maybe even three.
"Wait, I Feel It In My Triceps!"
The long head of the triceps is also involved and could take away some stimulation from your lats, so you really have to put your mind in your lats. Only use a weight with which you can focus the tension on the lats. Keeping a very slight elbow bend (just enough so that they aren't totally locked) and never changing the elbow angle during the set will also help minimize triceps involvement.