Practice practice practice.... but I get the feeling you are at a dead end when it comes to scratching. I have been there too. What helped me were some instructional videos from Q-Bert (arguably the best turntablist/scratch artist of all time).
Here are some of the ones I found:
Chirp Scratching (you probably know, but still...)
http://www.youtube.com/watch?v=Iwj5-9AD10U
Crab Scratching (a bit more advanced, the basis for most of the advanced tricks)
http://www.youtube.com/watch?v=R_6fVOEitTE
Transforming (yet another basis for more advanced moves)
http://www.youtube.com/watch?v=zmVhTFChkWI
Phasers:
http://www.youtube.com/watch?v=KsX4x4dyUT8
Dicing:
http://www.youtube.com/watch?v=ASjJCgI3cCs
Waves (basic scratch technique):
http://www.youtube.com/watch?v=SN8eaNGOUpA
ZigZags:
http://www.youtube.com/watch?v=0XMRNAwNjUY
On the zig-zags video, if you look over to the right, there are a ton of "related" vids that are in this series that teach a crapload of different scratch techniques.
This is how I learned most of my scratches, but practicing and finding your own patterns is what its all about. These teach you the basic techniques to do a scratch, but it's up to you to own it and make it yours. Play with them, combine them, do them silly-style. Whatever you do, do it the way YOU want to do it, and I promise, you will love the way it turns out.
If you can't do some of these, don't worry... nothing worth doing is easy. Just keep at it, don't give up, and eventually you will be sitting back going "Damn, did I just do that??" and you will all of a sudden "get it".
Hope this stuff helps!