When wiring a rack there are some pre made cables which you have to use things like HDMI leads, come in 0.5m, 1m, 2m, 3m, 5m etc.. but if you need a 4m you have to use a 5m and are left with 1m of extra cable, whats you technique to lose / hide it so it dosnt get in the way ?

Something like a cursive capital U, keeping an eye on bend radius.

Neat patch

Usually a couple of circles on the side or under/over the equipment

Ever hear the old saying, go around your ass to get to your elbow? Take the long path. Put a laced bar above and below the equipment so you can run a loop

I use a lot of snap HB-1U's for hiding small pieces of equipment. I utilize the bottom of those shelves to tie coils and transformers. Hides the ugly, still there though.

If you're using wider racks with side panels, you can hide some service loops on the sides (power on one side and signal on the other).  Most of my jobs these days are at least two rack so I use Middle Atlantic BGR racks and join them together with the optional cable chase which adds 4.5" between the racks with removable covers.  You can hide a bnunch of stuff in there and it's really easy to access.