Prestige

Which things affect prestige? I have a solid economy and several prestigious titles, yet have a lower ranking than several people with fewer titles and lesser economies. What else goes into the prestige calculation?

About titles, only the highest rank matters. If you have 10 Viscounts and 1 Duke and another one has 1 Viscount and 10 Dukes, you have the "same prestige" (according to the prestigious titles), because you're both Dukes.
The second aspect is about your economy. The more you're rich, the more you're prestigious.

After, you have several bonus/malus, due to your marriages.
Maybe prestigious characters affect prestige, but I had one, with a small amount of money (to test) and I saw no change.

And finally, your prestige is relative to the others. So calculations are difficult. The idea is, your prestige moves according to the prestige of the others. You have not really a value of prestige, it's more like a rank. If someone becomes more prestigious than you, your prestige value can decrease (even if you've done nothing special).

There is a legend which would say that even games creator doesn't know how is it calculated but....hush.. 

The reason I am curious is that I just got a Count title, yet my prestige rank went down, not up. I haven't married anybody off or changed anything else - so I suppose it could be relative, but I would think it would go up first. 

Don't remember for sure, but your prestige is calculated at the new year. Except for marriage bonus/malus, it's immediate.