Wood Heaters, page-190

We havnt cleaned the new flue set up for nearly 6 years but when we used to, we used a piece of small chain (6mm) works well. Swing it around at the top in the same motion you would stir a bucket of paint and it makes a spiral motion down the flue. A couple of minutes is all it takes.
Adding a larger piece of wood occasionally once you have a good coal bed with the smaller pieces will slow the fire and reduce the need for topping up without creating smoke also.