I bought my first saddle about 5 years ago. Lately I've had problems keeping my leg back and my seat being too far back on the saddle. I have also noticed that my knees go over the front of the saddle as well. I read somewhere that a good fit meant you could fit your whole hand behind you when you sit.
I'm wondering if perhaps I have grown out of my saddle, since I've recently (within the last year or so) noticed these problems. I do have long legs as well, and have been known to hike my stirrup leathers up while riding some of the smaller horses, as I have been lately. However, I still notice the knee going over the saddle whilst riding with longer stirrups on the taller horses.