I would suggest trying after-market insoles like Superfeet. They tend to make the heel fit a little tighter. If the shoes don't fit, socks really won't help the problem. However, the BEST socks are a good merino blend. I'm partial to Smartwool. They make them in everything from thin dress socks to super-thick hiking socks. They last forever, keep your feet dry and odor-free also.
If you have an REI or a local camping/outdoor shop they will have these brands. Also, these places may be able to fit you for better shoes.
Also, check out the link before for a different way of tying your laces. This can work WONDERS for hard-to-fit feet. Lacing this way basically locks the laces in place more, keeps the foot back in the shoe how it should be and is more sturdy. Try Method #1 on the site.
Ian's Shoelace Site - Lock Lacing