I've been cycling in NYC all of my life. Bicycle theft is real here and it sucks. Most bike thefts are a crime of
opportunity. So don't give those bastages the opportunity to steal your bike.
Get a Kryptonite NYFU u-lock and a Kryptonite NY series chain. Yes, I said
both! And you'll want to have a seat lock installed or use a cable lock to lock that down.
Having two different types of locks on one bike twarts most attempts at theft. Only the most well-prepared, pro thief with a LOT of time on his hands will even have a shot at taking your bike.
Now that you have the best u-lock and the best chain on the market, locking technique is critical!
1. Pick a busy, highly visible spot with other bikes locked to the same bike rack.
2. DO NOT PARK YOUR BIKE IN THE SAME EXACT SPOT EVERY DAY!! Mix it up.
3. Lock your bike next to more expensive bikes that aren't as well protected.
4. Place the locks OFF THE GROUND and as high up as you can on the bike/rack.
5. Take up as much slack as possible as to not leave lots of space for thieves to use their tools.
6. Run the chain through the rear wheel, the frame and around the bike rack then the u-lock through the front wheel and the frame -- if you can fit the bike rack into the u-lock along with the front wheel and frame, that's even better!
7. Take off any lights, cycle computers or anything else that can be easily stolen.
8. Check on your bike throughout the day, like during a lunch break or between classes. You might even want to move your bike to another spot.
9. NEVER LEAVE YOUR BIKE LONGER THAN IT TAKES TO BREAK THE LOCKS!!
^^^ There is no such thing as a theft-proof lock! Not even a Kryptonite NYFU lock! All locks do is buy you
time. If you do as I stated above and get both the Kryptonite NY u-lock
and chain you've bought yourself a good deal of time but it hasn't bought you an eternity! Check on your bike frequently.
If you can, bring your bike inside with you and no matter what you do
please,
please do NOT leave your bike locked up outside overnight. You've just given the thief more than enough time and the cover of darkness to work on breaking your locks. If you lock your bike up outside at night, do not be pissed when it's not there in the morning or parts are stolen off of it by morning.
My last bit of advice is ride a "beater" or "uglify" a nice bike. The less attractive the bike is the less it's worth to the thief. It's harder for them to fence the bike if it's ugly or looks seriously beat up.
Personally, I ride a folding bike that I take with me inside 99.9999% of the time. I also own a full-sized bike that's a vintage (nearly 30 years old) beater. It rides like a dream, though you wouldn't know it from just glancing at it. I use a Kryptonite Evolution Mini u-lock and a Krytonite Kryptoflex 4' cable. (I have a "trick" back. Weight and bulk were of major concern for me so I choose a slightly lighter weight lock.) Both of my bikes are bought inside at night and I never, ever leave either locked longer than it takes to break the locks.
Best of luck to you and your bike.