It's not always possible to buy the plants you want (at the price you want, when you want) from a tank set up similar to yours, but it's nice when you can. I run into major melting on a lot of stems as most of the plant sellers out there farm under high light/C02 to get fast growth--and I'm running low-tech/moderate light tanks.
Other than that--being very careful to bring the plants to temperature very slowly is the best preventative I've found. If the interior/exterior temps are very different, I'll bring the box inside and just let it sit for awhile to start to equalize to room temp, then float the bagged plants in room temp water for an hour or two, and only then float them in the actual aquarium.
And, of course, handling them gently. If the roots are too long to handle or there are damaged leaves, trim them up before you try to plant them so you aren't having to manhandle 'em repeatedly.