I’ve always believed that a conversation is more like an act or a performance: it is not only about knowing the words or being good with grammar, but rather the way you approach people. And this has a lot to do with the local culture, don’t you think?
You’ll find many words for greetings in any phrasebook (and you can sure pick up something like “Здравствуйте!” (Zdravstvuyte, Hello -- Rus.) or “привет!” (Privet, Hi -- Rus.) by just hearing it), but there’s something about starting a conversation that can be of much help.
“Прошу прощения” (Proshu prosheniya) literally means “I beg your pardon” and apart from a very polite way of saying that you’re sorry, it’s also good for getting someone’s attention, like when you want to order something, inquire for information or even get through a thick crowd of people without getting much physical contact. Basically, it’s a real lifesaver in just any kind of a situation, and in case you forget how it sounds, remember that you can just use the word “Pardon”, stressing the /o/, even though that’ll be quite a fun way of putting it.
“Секунду” (Secundu). You’re right: that literally means “a second” (which makes it really easy to remember), and is a way of saying “give me a moment”. Trying to converse in a language you’re not used to can be quite hard, and if you’re feeling that you’ve lost the thread of a conversation, saying that can give you time to catch your breath.
“Увы” (Uvy) means “It’s a shame” or “Regretfully” and may be a bit hard to pronounce, so try asking your Russian peers to learn the proper way of saying it. What’s the point? Well, this short word can help you out of a conversation you don’t understand or don’t even want to start; among other things, it works well for saying “sorry, I don’t speak Russian” or “sorry, I would’ve liked to talk but I’m really in a hurry”, especially if some stranger approaches you in the street. Just slightly raise your hands with a mild smile, say “увы”, and be on your way: this way, you won’t be offending anyone.
Finally, a couple of general tips. A lot has been said about Russians being somewhat cold and gloomy, but moods change much like St. Petersburg’s weather. So, don’t be afraid to speak up and approach people: most will at least try to understand and help you. Also, it’s practice that makes perfect. If you forget or don’t know some word, don’t hesitate to look it up, or even doodle: this will at least make your experience more memorable and fun - and hopefully help you learn some more!