Don't whistle in the house. Whistling in Russian homes is believed to cause money problems. Possibly a remnant of when people weren't allowed to blame money problems on Communism.
Don't mix your vodka. If you're offered vodka, never ask for a mixer. That's seen as an insult to the national drink, and a sign of weakness on par with riding a horse WITH a shirt on.
Don't cross your utensils. In case you haven't ever watched a mob movie/been to Hoboken, the holy cross is an important symbol to Italians. Placing your eating utensils on top of each other at perpendicular angles is seen as an insult to the religious symbol, and bad luck.
Eat whatever is offered. Doesn't matter if it's a curry you're pretty sure was made with battery acid. You accept it, you eat it, and you compliment the chef between your tears.