Make sure you secure every cable with multiple zip-ties, etc. Avoid excessively heavy cpu hsf. Stock fans are the best for this. Use 4-8 screws for each drives if possible. Lastly, ask someone at the other end to double-check the contents before powering up. Especially someone with diy pc knowledge.
You might also go one step further by removing heavy cards such as GPUs and soundcards and shipping them in the original boxes as well. Take some pics of 'where stuff goes' and provide the information on how to seat the cards when the box arrives. This will do 2 things, 1) these components will survive the trip much better in their nested boxes of cozyness, and 2), it might just spark enough interest for them to want to build their own next time.
Ditto on the using OEM HSF for CPUs and such, if they are having you build/ship across the country, I doubt they are going to overclock themselves when they get it. Avoid those huge tower coolers, they might just come unsnapped or loose and it would just be a hassle and headache to walk them back through re-attaching the sucker.