It's your bike... keep it that way!
A few things you should consider when choosing a lock and tips for locking it up:
1. The level of security you need depends on where you lock it, and how long you're locking up. The longer you leave it locked, and in more populated cities- you'll want to ramp up the security with a heavy-duty locking system.
2. Lock your bike to something that cannot be lifted or cut through! Seems like a no-brainer, but check the object your locking to to see if it is loose or easily moved.
3. Lock your bike in high-traffic, preferably well-lit areas where they are least likely to be tampered with.
4. Try to lock your bike through the frame AND your front wheel too! If you can get it through the rear wheel, that's a plus... Extra cables can help with this puzzle.
5. Do you prefer a combo lock because you can never remember your keys? Never lose your keys with a combo lock...
6. To extend the life of keyed locks, make sure the key hole is facing down so it is less likely to pick up dirt and dust.
U-locks, which combine a solid metal shackle with a crossbar, have a reputation for heavy-duty security due to their rigid structure that can't be cut with a bolt-cutter. Though they are sometimes awkward to carry and the rigid structure can be tricky depending on what you're locking to-- they can be combined with a cable to weave through multiple touch points on your bike, like through the wheels, for maximum security.
These sleek fold-up locks do a great job of combining form and function. Folding locks and are easily transportable as well as lightweight. They typically come with mounts that attach easily to your frame, so you don't have to remember to take your lock with you every time you ride.
Though these bike locks can get heavy, chain locks allow maximum security as well as flexibility to be looped around to virtually any bike and/or object. Chains can withstand high-crime areas as they are tough to leverage and are very difficult to clip.
Cable locks are lightweight and easy to carry which are suited for a light-to-medium level of security. While they are great for day trips or locking in low-crime areas, cable locks are more easily clipped, so make your stay relatively short with these kinds of locks.