Neil,
It all comes down to three things.
1. The selection of a quality paper.
2. The proper clearing of ALL ferrous and ferric conmpounds.
3. The removal of all clearing compounds from the paper, and possibly mild buffering.
The paper selection is fairly easily made, as long as you are using a good quality printmaking paper or any of the traditional pt/pd printing papers you will meet that requirement to ny knowledge. Don't try to print on newspaper...
The removal of the iron is the hardest to do, and frankly, most people do not do it enough. It's actually very difficult to remove all the iron compounds form many papers because of the buffers in the paper. Many people attribute substantial warmth to palladium, and while it is warmer than platinum, I think you may find that much of the warmth attributed to palladium is actually connected to the iron, which seems to be a bit more tenacious with the pd salts.
Hold a print that you think is cleared up to a light table and look carefully. The highlights should be just as white as the paper edges. If not, you haven't removed all the iron compoounds. Some people look at the print from the back side on the light table. It is often apparent if there is still iron in the paper.
The removal of the clearing bath solutions is important because many people use some form of acid in clearing, and the acid will eventually yellow and damage the paper with age if it is left in the paper. Some people will do a final bath in a buffering solution to account for this, others will wash the print a very long time.
If you have a PH pen, you can see if the paper comes out acid or not. I once did a test where I checked the PH of an acid cleared and then washed paper to a traditional sodium sulfite cleared paper. Even with the recommended 30 minutes washing, the acid treated paper came out showing some acidity remaining in the paper. SO, it is probably a good idea to always have the last clearing bath be a buffered bath of some kind before going into the wash.
---Michael