Originally Posted by fourknees
To secure your bike without a lock, if you have a road bike, put your brakes in the "open position" and adjust them for braking (before you are out for a ride). Then when you need to leave your bike on a ride and go inside, put the brakes in the "closed" position and it will clamp on to your wheel.
Couple this with putting the bike in your very highest or very lowest gear, someone won't be going anywhere very quickly.
This won't keep the bike from getting stolen as it can be picked up and carried, but it will slow down someone who thinks they'll just jump on and ride away.
I think it would be easier to just remove your front wheel and take it with you. Like you said, it could still be picked up and carried, but you wouldn't be able to ride it away at all.