Flingwing207 has it right on. The problem with wandering rpm in autorotation is almost always that changes in the inflow of the rotor are making the rpm climb or drop. The cyclic can be as powerful as the collecive.
The way to make the rpm stay solid is to keep the pitch attitude of the aircraft steady, which keeps the disk at the same relative angle of attack. As Fling says, keeping the cyclic steady, which keeps the nose steady, is the key.
One thing to feel while you descend is the vertical G, which tells you exactly what the rpm is going to do. If you pull the nose up, to build a little load factor, and also increase the inflow. The inflow makes the rpm rise, and the loadfactor is your clue. After some practice, you can judge the collective up that is needed by feeling the load factor, so a slight increase in load factor calls for a slight increase in collective, and vice-versa. This load factor sense is also helpful in turns, as the load factor is what makes the rpm rise in turns. My experience is that most students are ok at triming up on collective on turn entry, but they plumb forget to lower the lever when they roll out of the turn.