I assume that this is leakage that happens with normal use. I have found that with many single handle faucets, if you push the handle all of the way open, hard over and hold it there, they will all leak.
Turn off the water to your faucet from under the sink. There should be a shutoff down there. Make sure that both the hot and cold water lines are shut off, so that when the faucet is open, no water should come out.
Open up the faucet by removing the handle. Usually this is easily done with a screwdriver. There will be a screw someplace to do this with, although it might be an allen type set screw. Sometimes that screw is neatly hidden underneath a plastic plug.
Pull out the cartridge. Drive to your nearest home center, with the cartridge in hand. Now it will be trivial to find the exact cartridge. If you are still unsure, ask for help.
Buy the proper replacement cartridge, complete with all o-rings, etc.
Replace the cartridge into your faucet with the new one, replacing ALL o-rings, all of the pieces that come with it with the new ones.
Put the handle back on, turn on the water.
If it still leaks, buy a better grade of faucet. I have found that cheap hardware sometimes really does perform like CHEAP hardware.
You say you replaced the seats. The seat is where the usually rubber washer on the end of the stem seals against the back of the valve body. If you put your finger in the hole where the stem goes you should be able to feel the surface where sealing takes place. This surface can get scratched and allow leaks. In fact, the leak can make the scratch worse over time. Also, debris can become trapped in this space during repair and prevent sealing. So, make sure the seat is not scratched, it can usually be replaced if it is, and make sure no debris is left behind before closing up. Additionally, there is usually a washer made of paper fiber or hard plastic that sits between the valve body and the stem above the threads. If this washer is worn or missing leaking can occur at that point.
Finally, you say you replaced the seats and it got worse. It is possible the seats are not tight or are installed incorrectly, crooked, or cross threaded preventing the washer from seating. Check for that.
Good luck!
Best Answer
(sorry about not knowing all the names of stuff, the english is not my native tongue)
Depending on the type of the faucet, there's an option for a really cheap and more permanent fix. We had a leaky faucet and each time the 'rubber-part-that-goes-inside-the-faucet-handle' (sorry, don't know the real translation :)) gets replaced, it works for a little while, but after a month or so you just can't turn the faucet hard enough to stop it from leaking.
I'm not sure if that's the problem you're having, but here's what was the problem in our case... the place where the faucet knob/handle goes in, if you unscrew that just like you're preparing to replace the rubber (provided you closed off the water in bathroom), you will see where the rubber on the inside of the handle touches the metal circle. The turning of the knob is what moves the rubber to and from the metal circle, giving the way of water to pass through. The problem here is that if the water is dripping for some time, the metal gets "eaten away" by the water, ie there's a canal going through it's surface, so no matter how hard you turn the knob the water passes through the canal. When you replace it the rubber is more flexible and it can fill in the gap, but after a while it hardens and that's when you need to replace it again.
The only solution here is to take off some of the metal. There's no way you can do it without a proper tool though - there's a special "screw" with metal teeth that you tie instead of the knob, and by turning it like a knob the metal teeth eat away at the metal evenly. That way the metal gets even, and now when you place your original knob it will hold the water in.
Hope it helps.