There is no secrete to neons, more like a 50/50 chance. Sometimes the LFS gets a good batch of "neons that never die" other times the LFS gets a very crappy batch of "drop dead neons." I've worked for a LFS, and trust me, neons are just one of those fish that either "work" or "don't work" depending upon 1000 different factors.
Next time you are in the market for neons, visit several LFS and ask them what their turnover rate is on these fish (I used to sell out of 100 neons in a weekend, I probably could've sold even more if I could keep them stocked!). Ask them where they are getting their fish (wholesalers like Segrest Farms use FL based "bulk breeders" while others might import wild fish from SA or use Asian/SA breeders). Home grown fish are more likely to be stronger. Ask them what day the fish arrive, then purchase the neons at least next day. If possible and the LFS turnover is slow, show up every day for a week to monitor how their neons are doing in the store. If for any reason you don't like that store or that batch of neons, don't buy any, not even under the mistaken pretense that they'll do better in your tank once you get them home. Also, beware of the season. Mid to late spring is a bad time for neons because they are spawning, and the breeders are shipping "exhausted" fish to keep up with demand and make room for new spawns. These exhausted breeders suck.
Good luck.
~~Colesea