What's the most important thing or best way to get more fans to join in a spot? Banners? Icons?

I would say that the icon and banner are most likely one of the most important parts. I get turned off joining a spot, if it doesn't have an icon or banner, it just shows that the person who created it didn't really care, and couldn't be bothered with it. It also makes it a lot easier for the fanpopper's to find the spot on their profile page to click on.

When I first make a spot, I will also go through, and add at least 4 pictures, 1 fan pick, 1 Pop quiz question, and maybe a forum entry, or an answer question. I also might add a link.

I also go to any spots that are related to it in some way, and add a link to the spot I just created, or something to let people know it's been made.

Of course, the MOST important thing in getting people to join, is creating it about something that people love! (This is hard, because normally someone has already made a spot for all of the best things)

Hope this helps,
Stevey

Truthfully, the subject is the most important. I'm not going to join a spot that I don't have any interest in. If you want people to join a spot- pick a topic that you think they'll be interested in and not something that hasn't already been done.
If there's no icon or banner and I care enough about the spot- then I'll make one for it and suggest it to the creator. If there aren't any interesting picks, I'll make one.
It all depends on my interest in the subject.
To get people to join I think is more about advertising the spot. Get the word out to people who might be interested in it. Post links on related spots and get other fans to do the same.
Here's a link to an article about link and one to a forum where you can link

The banner and the icon play an important role but ihonestly think that the first picks on the first page of the spot are the ones who truly make a difference.