^ how long have you been doing it?
I'm about 4 weeks in, and I can now do the Na1 with the Qd5 exercise all in my head, without board. I still need to concentrate and put a real effort though, it doesn't come automatically as it should.
I reckon it will take me another 2 or 3 months of hard work to be able to see things in my head istantly. But I'll get there.
I don't think you need to purchase anything. Just find a quiet room with no distractions, close your eyes, and try to build a board in your head.
I have divided the board into 4 segments and see patterns within each of the segments. A "dark tank" pattern, with a1, a3, c1, c3 as the "treads," b2 as the "body" of the tank, b4 as forward gun, d4 as the corner gun, and d2 as the side gun. The dark tank pattern is to the left, the light tank pattern is to the right, and is the exact inverse, with c2 as the body, etc. etc. The pattern the two tanks create is the same for each quarter segment of the board.
For the diagonals, I see what I call a butterfly pattern. Dark butterflies and light butterflies, and it depends on what direction your going. The four segmented dark butterflies move from left to right, the double three segmented butterflies with subby wings (only on square) move from right to left.
All of this though is just an aid--the trick is trying to see the whole board, so that you know for example e5 is a dark square with 3 squares above, 4 squares below, it's to the right if you're white, and to the left if you're black, and you can sense the b8-h2 and a1-h8 diagonals. Also helps to know that d4 is the brother square, or the inverse square. I'm still mixing up the names of the squares as black and as white, I guess it just takes a lot more practice. I'll walk a bishop on the dark squares in my mind for reenforcement, walk a rook around for the ranks and files, I'm not seeing the Knight squares well enough yet--I don't want to have to "calculate" them, I want to see the image in my mind. When I do this--I have a board handy and compare my mind's visualization with what I see. I'm still getting things wrong. I imagine this is going to take a long time, but the dark squared "tank" pattern is rock solid. The diagonals are still fuzzy to me. I do this at odd times during the day. Take a 10 minute "chess break." The most frustrating to me is that while I am seeing the patterns in my mind's eye pretty well now--I keep screwing up the names of the squares. It's maddening. I do the same thing at tournaments, which makes the games unreliable.
A check is that "heavy squares" are always dark (Odd-odd, or even-even). Mixed squares are always light. For example a1--odd, odd, dark. e4--odd, even--light. I just use that as a check because sometimes I'll shift in my minds eye--say "e4" for instance when I'm seeing e5 (the dark square) What I'm seeing in my mind's eye is actually e5--I'm calling it e4 for some stupid reason. The images are clear, it's the names I keep messing up. I don't know why that is but I really want to fix it, because I am sick and tired of messing up my score sheets, rendering them useless. I was thinking of purchasing a Monroi--but that does not reallly solve the underlying problem.