Long term or short term advertisement offer?

I run a website that attracts university and companies that offer program, service, software in a certain niche.
I'll plan the site in a short time and have several potential clients contacting me about putting up a banner.

So as I think of how best to attract and retain future clients, what do you think is the best way to offer the advertisement options for them

1) on premier banner ads, we will have stationary and limited spots
2) we have other rotating spots throughout the site.

If we allow clients to lock in long term contract (6 months or more), it would be difficult for future clients to get a spot in our premier locations. Should we give discount for long term contracts?

Anyone can give some suggestion on what would be a flexible solution for this.

My personal favorite is to have a few, larger customers with whom you have a nice relationship.

You can charge them more in exchange for solid positions in the site. Instead of being just another rotated ad, they can be the "sole sponsor" of a certain page, or the "main sponsor" of your site with special rights.

You can time-limit all this so that if in 6 months you have dozens of people who want to advertise you can change things up. But at first that won't be the case, and this will reward those folks for being first.

I agree that having more people paying less each is a more stable cash flow than what I'm proposing, but at first I think my way is smarter.

Setting up different levels of sponsors seems like the way to go. Jason's point about having long term relationships with a main sponsor would be the highest level of sponsor (like Platinum or whatever).

I think the different levels also allows some up and coming sponsors to get their "feet wet" and see how the return on their ad dollars.