There isn't a tried and tested method that works, having trawled the internet some things work with some cats, but not one thing works with all, these are some things I've tried with varying degrees of success, anything that smells citrus is good but there's always one or two cats that ignore it. Having put bark down to cover the garden then two weeks later covered the lot with gravel out of sheer frustration I now have blue gravel with intermittent plants strewn with orange peel and shiny things. In short it isn't what I wanted my garden to be.
Here's the list of some things that have worked to a degree - olbas oil on teabags, bits of orange peel, half filled bottles of water (big lemonade bottles) shiny side up CD's, citronella, kebab skewers in the ground. A quick search around the internet shows how frustrating this is for people however a lot of cat owners are non-plussed about what they see as a fuss on our parts.
I've got to the end of my tether with this and found several e-petitions which are all outrageous or unclear, allowing culling of cats, allowing poisoning, stuff like that. I've set my own up which is that cat owners should be held responsible for cleaning the mess up and any damage their animals cause. I'd like anyone who's irritated by this problem to share it and sign it. It seems lots of people want something done but as a group we can't get organised at all so it falls on deaf ears.
I'd like to point out I like cats, mine have always used litter trays. I like wild birds as well. And can we buy a bell for our cat if we have one that goes outside. Please.