I have a really hard time with this too. I find the ladies at a couple of my local stores are really good at helping with this. Other stores, not so much, so you have to find the right one.
What sometimes helps is to pick a "focus" fabric - one with a combination of colours that you like. You know those go together, because they are in the print. Then you can pick others (and I usually tend to tone-on-tone) to match some of the colours in the print. Then, in the end, decide if you want the print at all - you still know you have a set of matching colours.