I sometimes had this issue a long time ago, but it was usually caused by a lack of focus. I have 3 suggestions, 2 are short term, 1 is long term. First, try counting points, not pieces. "We are even now, if I...I am up 3, down 2, down 1" just doing the math in your head for each piece where you left off. Second, after you calculate a line, just look at the board without calculating, and see if the pieces that you expect off the board are even or not. Third, practice on tactics trainer.
Calculating Material Gains/Losses
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i usually only try to count the number of "points" taken for each side.
in any case, i'd guess that just trying to "get better" helps... i don't remember ever trying to improve specifically in counting. :P
this isn't meant as a tongue-in-cheek response - it helps because, generally, "getting better" = "knowing more patterns" = easier to think about positions.
In a series of exchanges/tactical blow trading, I don't actually count points. I just visualise the position after all the fireworks are over (or after my brain can't keep up) and count material on that virtual board. Just like how you'd count material (pieces and pawns, not points) on the board in front of you, just inside your head with a different position on it.
I also no longer think in terms of points, but pieces, like thinking "R+N roughly = Q", "B+N = R+2P" or some such. It's like beginners pairing off captured material to see who's ahead, just only with pieces on the board instead.
I have a very strange problem in chess and I want to know if other people have this issue. To put it simply, I can't understand the outcome of a series of exchanges or tactics. I can calculate, move-by-move, what is going to happen in a sort of "I go here; he goes there" kind of way, but when I'm done doing so, I often have no idea how much material each side has gained and therefore I don't know who gets the better out of the deal. I want to know if there's anything that can be done about this.
By the way, yes, I have tried literally counting and saying "at this point I have two pawns and a knight for a rook" each time but in long series of exchanges I often manage to goof the count up and not realize that I'm actually losing material.