Every month or so, I do this variation to this leg press to shock my legs.
You need 2 ppl to help u. Load up the machine with a weight you struggle to do 5-7 reps with. Immediately after you have done a set, your spotters quickly take some weight off and you start again, this time going for more reps. You usually only have a 3 second break between each set.
You do 4 sets in a row, each time with a lower weight but with more reps. When you're done, your heart will be beating like you just sprinted 100m, and your legs will feel the difference the next day.
Oh, and dont lock your knees at the top of each rep. You get a better burn.
My legs have yet to plateau, even though I only seriously started to work them into my routine in the last few months. I'm not sure if that is y, but give it a try.