In reply to JemG:
I initially used electrical tape on all of my gear not knowing about anything else. The tape has slowly worn off and was a bit of a pain when it flaked off but served its purpose whilst using a mix of uni club kit and my own whilst building up my rack.
Replaced now with nail polish which is much better. Quicker to apply. Works really well when applied on the inside groove of I-beam shaped carabiniers, no wear at all. Have to be a little more inventive on some other pieces of gear to find a place not subject to that much wear but once you've used your kit a bit it's easy to find a spot with no scratches. Its less obvious at first glance compared to tape, but I know where to look for the paint that identifies it as mine.
Haven't bothered marking slings in any way at all. Whereas a lot of my mates all use similar dmm hardware, there's a good variety of different colour/pattern slings that there isn't a need to mark them.