Do you run a URL shortening website or use one? If you run one I would vote for Pay Per Deal ads - we run our offer on the affiliate network that have some great PPD offers that pay min $1,000, often $10,000 per single deal (mine is traditional PPS though and doesn't pay that much ). So, basically before redirecting people to desired page insert a PPD add by matching keywords in the title with keywords in the offer or so... If you need help with figuring it out, PM me.
Put Google Adsense on the main page for people to see when they create their short link. For the re-directs, open them in a framed window, where you put your own iFrame up on top. You can get some CPM offers ranging between 2-5 USD for that kind of traffic. That way you make more money the more your service is used.
If you have your own affiliate links, you can cloak and shorten these links by using either free service and domain. Free service - tinyurl.com Domain - you need to register a domain, it is not free.
Try Chitika. Chitika (pronounced CHIH-tih-ka), is a full-service on-line advertising network serving over 2 billion monthly impressions, across more than 30,000 websites. Chitika is also an excellent alternative to Adsense. David Jackson
set up a donation link. if your service is good then you'll be surprised at how many people are willing to support you. lots of sites do it.
It is really not easy to monetize a URL shortener site. As far as I know, the best way to generate income with a URL shortening service is to put the AdSense ads on your site, which is the most common way to make money for this type of site.
ya i guess it is hard to monetize it. Im actually trying to provide more and more services for my hosting biz (see my sig) like an url shortener. but ya i think adsense would be best for it. and after that, popups would prob do good too. cuz you could sell the iframe traffic ....