i think the key is starting simple and working your way up.
i started with chevrons, which turn only forward, then moved onto diamonds and variations thereof. diamonds and such turn forward and backward equally.
say, if you have your cards set up to make a diamond (i know how the colours are set up but i find it difficult to describe) then you turn so many turns forward with all the cards, then so many turns backwards with all the cards. it soon becomes simple to see which way you're turning as the diamond either closes or opens depending on which way you're turning.
once you've mastered that, you can move onto patterns where certain cards turn in one direction all the time, and certain cards turn backwards equally to turning forwards. two such patterns are Ram's Horn and Vine, both of which can be found at www.theloomybin.com
, which is an excellent website with a wee java applet for saying when to turn the cards back.
after that you get your much more intricate patterns like birka and snartemo, but i'm not going to go into them cos i can't do them myself and this has been my main hobby/money-on-the-side thing for over two years. mind you, i'm not very adventerous
i think the loomy bin is good for simple card-shifted patterns like Ram's Horn, but i don't think it does very good at explaining patterns like birka at all.
a good set of patterns to try getting into once you've mastered diamonds and all the variations of pattern to be found with turning all the cards in the same direction at once in the same pick, is Egyptian Diagonal. These are very simple, and a wide range of icons and effects can be achieved with this method. It's also very good, because it's good with a two colour warp, which is also the ideal warp for diamonds, chevrons, waves and doubleface patterns, thus you can work little bits of comp[lex design into a large length of simple band. this is nice if you want a lot of braid done fairly quickly but also want to show off a bit of skill
anyway, in short: start simple and practice. if it goes wrong and you can't turn back, just rest the cards to their original pattern and put it down to experience. all the bands that i've made serious mistakes in end up being used as baggage straps or dog leads as well as cords for hanging shafts and carabinas for weaving (i have a peculiar style of warping and working that involves a lot of ropes and carabinas)
practice practice practice. and take a break if it's pissing you off