I'm pretty sure it's already been posted, lsw!
But it can be hard to find info in amongst the nearly 2 million posts on here, so I'll bite.
I'm sure someone more senior will post a better response than me shortly, but here goes anyway:
Some premium factors:
extension - .com is worth more than .info for example
characters - aaa.com is worth more than QZX (probably!)
generic - non trademarked, meaningful, one maybe 2 words
Maybe check out the price guides on 3Character.com