be patient. It takes a while. I raised my cadence this spring from around 90 rpms to about 110. It took well over a month for the pace to become natural. When your legs are tired, they will naturally want to slow. But, that's just where you have to be even more disciplined.
As I wrote up recently for a different thread (here slightly edited):
In order to raise your cadence, you could try "cadence intervals". For these, you spin higher than your goal for just 10 seconds. So, if your goal is 95, then try spinning at 120 or more for just 8-10 seconds. Then recover at a lower cadence for 10-20 seconds or more. Repeat for a total of 3-4 intervals. Do this routine 2-3 times per ride. But, emphasize leg speed not power or going much faster--just spin faster in a lower gear. If you are already at a good fitness level, I would even say that you should try to spin as high as you can during the intervals. You will probably start to bounce in the saddle at about 120. When I did it, I know that I hit well over 150. I still do these speed intervals from time to time.
Personally, I very much like my 110 cadence. And I would recommend something over 90 for just about any rider. I am working on bringing my hills cadence up to the same level--but since I can't get lower gearing for my bike (without paying too much), I am stuck until I just get stronger.