The only efficient way is total immersion. I suggest you don't go to Montreal if you want to learn french. Great, great city, but not for learning french. Come to Quebec city or any other city. Over there you'll just end up speaking english the whole time. If you can, go to the most remote place you can find
You have to adopt the following attitude...
Put your pride aside and don't fear making a fool of yourself; you will make mistakes, have a strong accent and say funny things sometimes. Deal with it and learn to laugh about it. Do not let this get to you.
Force people to speak french with you. Quebeckers (especially those under 30) are often quite good in english and they are eager to practice or to show off the fact that they are bilangual whenever they can. Even if you adress people in english, they will start talking to you in english when they notice your accent. You must ask them right away to adress you in french at all times. This is very important because it will happen all the time, even outside of Montreal.
Be serious about it. Learn new words everyday and put genuine effort into learning the verbs tenses. They are complicated but very rewarding once you've mastered them. Do not get discouraged but the complicated grammatical rules. People will understand you even if you mess up genders, verb tenses or adverbs. It not a big problem! You will perfect those aspects over time. Always remembers that even Quebeckers make a lot of grammatical mistakes when they speak french. Perfection is very long to achieve but a level that is sufficient to make yourself understood is easily accessible.
Stay positive and avoid thinking of your learning process as an impossible to climb mountain. Cripples climb the Everest. Anyone can learn french.
If you want to get ready for your immersion, I suggest using software such as Rosetta Stone which is based on an instinctive learning process. The software shows pictures of objects and actions, you hear a voice pronouncing the word, you see the spelling, you repeat the word and by doing so you learn french like it was the first language you ever learned. This method doesn't use translation! You never see the english translation of what you're learning but your brain makes the connection naturally between two words that describe the same concept.
When you feel ready, try to think in french and start watching tv shows and reading books. There are tons of high quality french content on the web. Why not take an hour everyday to discover this incredible source of entertainment.
Most of all, have fun doing it and make some friends if you come over here. People will be glad to help you. In general, we are very impressed and fond of anglos that take time to learn french
What I'm describing is the attitude I had when I learned english, spanish and japanese on top of french, my first language. 100% guaranteed ^^