It's all about how you act. Walk in with your head held high, knowing exactly what you want and as if you own the place, but be polite and pleasant to the staff and all will be well. If you have to ask a question, try to ask it sounding as if you already know the answer. Confidence (or apparent confidence) is the key to everything.
And if you get bad service, bloody hell, write and complain! Then you might get some vouchers or something, and the person who was rude (who should be wearing a name badge) will get told off.