First, X-ray machines will not de-magnetize cards. X-rays are a different type of energy than magnetism.

I always put my wallet in a shoulder bag and put that through the X-ray (while keeping a close eye on it). Although in theory, the hand scanners and metal detectors do not have a strong enough magnetic field to affect cards, it could happen if you were to get close enough to the wand or frame of the scanner.

Do avoid (as previously mentioned) the anti-security devices. These are basically degaussers (powerful electromagnets) that have to handle de-magnetizing the embedded anti-theft sensors.

One suggestion is to keep your most important credit card in some sort of plastic sleeve. Body heat and moisture can affect the substrate of the magnetic stripe on your card and cause it to break down and lose the attached material.

Also, card readers can also contribute to the problem. Depending on the quality and type of heads used in the reader, they may have problems reading some cards but not others. There's nothing you can do about this.

I also think the card issuer is a factor here, too. There are different quality cards in use. Some like Amex and DC, appear to be some of the best. Others, like my credit union ATM card are really cheap looking. My (former) commercial debit card was another problem card. I had to replace it about every 6 or 8 months because it kept wearing down. The amount of use compared to my other cards didn't explain why this was happening. It just appeared to be el cheapo striping on the card that would wear down quickly.