Is there any way to increase a weapon durability?

http://darksouls2.wiki.fextralife.com/Bracing+Knuckle+Ring

Decreases the rate durability goes down. To my knowledge...I don't think there's any other way to prolong equipment life.

If you are playing on PC, then durability is drastically affected by the improved framerate on this platform.

A reddit user has done lots of research into this, but the long and short of it is that a lot of actions in the game are measured in number of frames. As the PC version runs at 60fps, a number of these actions are doubled up. This mainly affect durability when you swipe through a fading out enemy (after killing them) but it also affects the animation speed of various attacks.

To mitigate the effects, you can use your graphic's card drivers to limit the framerate in DkS2 to 30fps, with the trade-off that movement isn't as fluid.

Use the brace knucle ring found in Lost Bastile