It's hard to critique form when we can't see you actually lifting the weight. Many people butcher both of these exercises.
For these moves to work you have to have proper technique.
For lat pulldowns:
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You could use both wide/close and forward/reverse grips.
Bring the bar down slowly and evenly to the top of your chest and then back up .
Keep your back straight and only lean back very slightly.
Don't lean far back in order to lift more weight and don't bring the bar lower than the top of your chest.
Don't bend your wrists.
Don't rock back and forth.
Try using different cable handles.
Seated cable rows:
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Sit up tall and tighten your abs.
You could use both a forward or reverse grip.
Squeeze your shoulder blades together as you pull the bar to your lower chest.
Don't round your back.
don't rock back and forth.
The most important thing regarding these two exercises is to concentrate on your back muscles . Many people use their arms to pull down the weight. It's critical that you use your back and not your biceps and other muscles to lower the weight.
You're arms should be fairly relaxed as you lower the weight . Almost all the effort should be coming from the back muscles. Feel the muscles contracting in your back. If you are feeling these exercises anywhere else then you are doing them incorrectly. Follow this simple principle and I'll guarantee you will see gains.
And finally, make sure you are using enough weight that your last rep is difficult. But remember , the heavier you go the more the tendancy to cheat and use your arms increases. It's upto you to find the right weight to maximize the effort your back muscles are making.
Ya, like Mudge says, Freeweight Rows are also very good.
And don't forget Pullovers. Experiment.
P.S: I've owned a cable pulldown/row machine for a while and I have to say that my entire back has gotten alot stronger since making it a big part of my workouts.