Use cables as close to the correct length as possible.
Spare cable should be coiled away from the concentrator - so spare power cable gets coiled next to the machine, not the powerstrip, and spare network cable next to the machine, not the hub.
Don't be stingy with cable ties, be they zip ties or velcro pulls. When in doubt, use an extra, and don't hesitate to trash old ones.
Machines should still be removable once unplugged, which means run your cables vertically down the edges of the rack, not the middle.
Only run cross-rack in one place (probably the top or bottom of the rack), and run as little cross-rack as possible.
Color coding is good, but when troubleshooting, what is trumps what's supposed to be.
Fix it all now. 'Later' never comes. Corollary: everything should always be perfect before you leave - or you have concrete plans to come back and fix it.
Label cable ends with abstractions: numbers, letters, whatever. Don't bother with 'firewall', 'external', etc - they'll get moved and re-used and the labels will be wrong; better to keep the labels an extension of the 'color' so they can be easily reused without having to be relabeled.
A lot of this is from old telco practice - you think we manage a lot of cables, you should look inside a real telco crossconnect sometime - well, but don't because they're often spaghetti.