I had a problem with this also. Here is what I do:
During my 3-2-1, I tell myself the following at no. 2: "Any outside distraction will cause me to get deeper into state." After a couple of times, my mind stayed focused.
Another thing I do, which was suggested by another poster, is to play music, preferrably Baroque or similar. It's not always practical, but it seems to help.
Anytime I get a distracting thought, I will thank myself for being concerned, but this is the time I set aside for pr and that I will come back to that distraction later. I found that if you get upset with yourself, the distractions get worse; much like a child, who, when being ignored, only gets louder. Actually, it was Paul Scheele who suggested this at one of the pr classes he taught.
I sure there are many other suggestions that others might have that will also work. Hope this helps.
--BReeves