Nothing is worse than returning to the location you locked up your bike to find that it is no longer there. In fact, to ice the cake, not only is the bike missing but so is your lock, and the pole it was secured to.
While organized crime professionals and other such thieves will always be out there eyeing up junky bikes to top of the line road bikes...there are ways to deter them from whipping out the wire cutters and wrenches.
1.) Lock your bike to designated bike racks.
Do not attach them to signs, random poles, small trees, or fencing. When you do attach your bike to a rack, make sure that the rack is not already loosened out of the ground. Pull up on the rack, and check the bolts for signs of tampering.
2.) Choose a smart location.
The ideal location to lock up your bike is a moderatly busy area, that is well lit at night, with regular foot traffic. Bad neighbour hoods, dark alleys, and crowded streets are generally higher risk areas. No where is 100% safe, but you can still use common sense and choose wisely.
3.) Bike cages are not thief proof!
Many apartment buildings and office buildings have designated bike cages that people can park their bikes. Often people don't bother putting a lock on their bikes in the cage, or they lock their bikes together loosely. Same rules apply for these cages, you should always use a good lock...and if the cage has been broken into several times before, consider a different option.
4.) Always use two locks and remove accessories.
The more locks you have on your bike, the more it will deter theives from attempting to crack the locks. Keeping a U-lock around the back tire/frame/bike rack and a rubber wrapped chain on the front tire/frame/bike rack is the safest. Remove all lights, horns, bells, and water bottle holders if you feel you are in a dangerous area. Often people just take these for the sake of them being there to easily remove. If you want to get super extreme you can even remove your back/front tire and bike seat (this is usually a good idea if you are parking your bike overnight somewhere outside).
5.) Record your serial number.
If/when your bike gets stolen, report it to the police. You will need your serial number to finish the reporting process. Often bikes have their serial numbers scratched off intentionally before re-selling them on the streets. There is no way to stop this, and it can lower the chances of getting your bike returned to you. Putting unique markings on your bike will help aid police in positively identifying your bike. Stickers, and secretely engraving your name into the frame of the bike can be good tricks.
6.) Invest in good locks.
Models that come with insurance are the best option, anything that has over $1000 insurance are generally the most secure ones to get. These are generally U shaped locks, and are heavy to pack around. You can get ones that can attach to your bike, or you can just carry it in a messenger bag. Get a secure chain or rubber coated wire lock for your front tire in addition to the U Shaped lock. The more secure the better!
If all else fails, consider getting a GPS tracking device for your bicycle. Expensive yes...but when your heart has been broken several times over, it might be a good final option. Then when it is stolen you can report its location directly to the police.