I separate as much as possible.  Using a larger (wider & deeper) rack like Middle Atlantic BGR can help compared to working with a narrow skeleton rack where you don't have as much room for cable management.

Using custom length power cables can also help vs. zip tying a bunch of 6ft power cables into small bundles.  I've been using Middle Atlantic Signal Safe power cables for 4-5 years now.  They're affordable, available in lots of different lengths and have twisted conductors to reduce the potential for interference:

[Link: middleatlantic.com]

Vertical power strips/distribution devices also help and will often let you use really short 1ft or 2ft power cables.