What filter system instead of screw on filters?

[cherokee1111]

I buy a new lens, get filters, sell the lens and have expensive filters hanging around or sell at a loss
so basically what can I attach to any lens when I want to add an ND10 or polariser (a screw on UV will already be attached)
sorry for the basic question but its all still very new

 

[Ploddles]

Cokin do a relatively cheap system that has different sized adapters to fit a variety of lens diameters. Loads of different filters then just slide in.