So far, there have been only two instances when I had to use Вы, once when I was at the airport and I met a Russian woman and the other is when I talk to my teacher. 95% of the time I am talking to my friends so I am using ты, but then when I talk to my teacher, I know I need to use Вы, but I always end up using ты (and ты forms of verbs) by accident.
I know that I am supposed to use Вы with elders and the like, but in an attempt to not talk slowly ты usually spills out unless I concentrate on using only Вы, but then my fluency is much worse. I am going to St. Petersburg in 4 weeks with some members of my class for an exchange program and I would like to be able to show respect where it is due, so how can I practice using them correctly? (Ты with friends and Вы with elders)