Hi,
I always read in forums like this one that you practise and practise and practise. I counted a half an hour a single deck, then 30 min pairs and last but not least 3 cards at the same time for about 1h. I did the same with 6 decks in a row and that's it.
I really don't understand why everybody is so afraid about counting decks. Dealers are so slow and even if they draw fast, the players decelerate the speed of the game that you'll have no problems to count.
I think most important is to see the pairs or even to "scratch out" pairs of cards which match on the table like:
Box1: A, 10 = -2
Box2 8, 9 = 0 just ignore it
Box3 2, 10 = 0 just ignore it
Box4 10, 9 = -1
Box5 5, 8 = +1
Box6 8, 8 = 0 just ignore it
Dealer Upcard 5 = +1
U see that Box4 and Dealer have +1 and -1 = zero so just ignor it.
Hope it helps a bit.