Do Your Homework
The best way to avoid a faux pas at IKEA is to know what to expect. A cornerstone of IKEA's business model is flat-packed furniture, which shifts the burden of assembly to the customer — and might cause you to have a panic attack if you were really counting on sleeping in that bed tonight. (But rest easy: For an extra fee, IKEA can put the furniture together for you.)
If you go on the weekend, it's probably going to be crowded. Even in the middle of the week, a trip to IKEA will likely take at least an hour — and that's if you stick to your shopping list and don't wander off to look at lamps.
Next, come prepared. Before you even leave the house, scan the catalog and browse ikea.com. Snap some photos of the room you're planning to furnish, and think about how new furniture additions will fit its character, context and color scheme. Next, break out a tape measure and calculate the size of the room — you'll save yourself the deep despair of assembling a bookcase only to find it's too tall for your study's low ceiling.
And instead of asking IKEA employees their opinion of whether that couch would fit in the trunk of your Volkswagen Golf (it probably won't), just measure it yourself first.