BJCP judges have to deal with this all the time. Crackers are a good thing, as is the water. And take your time. I'd hit up the BJCP website for suggestions. One thing that can help you taste is to keep practicing. Maybe sit with some others and all share opinions, and all search for that elusive flavor.
But everyone has different tastes, and different sensitivity to flavor. I think it's interesting to see the many different opinions on the same beer. Especially when the people all know the different flavor descriptions, but haven't been trained to spot them yet. Your imagination can make you taste things that nare necessarily there.
But just to comparison taste, I think the crackers and water thing is a good idea. And limit the amount of aromas that don't come from the beer, and try not to get distracted by stuff going on around you.
You may want to have some paper towels handy for those really bad beers, or soem Kleenex for those really good beers!