What conditions make xanthan thicken? I have used xanthan a few times, sometimes successfully, sometimes less so. The one thing I've noticed: it is certainly not enough to just stir it in until it is well dispersed. Sometimes it will not thicken no matter what I do, sometimes it will stay liquid for ages while I'm stirring and then suddenly become one big gloop with no taste left. And sometimes it will get thicker with whisking and then get back to liquid. 

Following factors could play a role: 


stirring 
whisking (actively beating air into it) 
time (maybe I have to leave it to hydrate on its own?) 
temperature: I have gotten it to thicken at room temperature, but does it work better or worse when hot or cold? 
liquid ingredients: How does the percentage of sugar, fat and other solids influence it? What about pH? 


Can somebody explain 


How do the above factors affect thickening, and 
What is the most efficient method to get a liquid thickened?
The viscosity of a xanthan solution is virtually unaffected by temperatures from
freezing point to boiling point of pure water and it hydrates rapidly in cold water. You don't need to let it sit on its own and the temperature doesn't matter.

The viscosity of a xanthan solution lowers when whisking or stirring, a process known as shear thinning. When you stop whisking, it will return to its previous viscosity. This will not factor in to thickening. That being said, you can get a xanthan gum foam using an immersion blender.

Between pH 1 and 13, the viscosity of a xanthan solution is practically constant. Unless you're using it for a strange application, you're good.

I cannot find any information on sugar or fat or other solids and their interaction with xanthan gum. Anything that can be dissolved in water would theoretically affect the xanthan gum's effectiveness, so I would expect sugar to be an issue, but fat or solids are obviously not.

The only thing you don't address is the concentration of your xanthan gum solution. Modernist Cooking Made Easy suggests a by weight ratio of:


0.2% ratio for light thickening
0.7% for a thicker sauce
and up to 1.5% for a very thick sauce


Additionally, the thickening effects only hold up for a day or two. What are you trying to thicken, specifically?

References


http://www1.lsbu.ac.uk/water/hyxan.html
http://www.lucidcolloids.com/pdf/xanthan.pdf‎
http://www.modernistcookingmadeeasy.com/info/modernist-ingredients/more/xanthan-gum