The most reasonable thing to do when canceling your istockphoto exclusivity contract is to have your entire portfolio uploaded, submitted, reviewed and waiting to go live on all agencies you wish to license your footage at. The reason to this, is simple. By going freelance you loose a considerable amount of istock profit. Bronze contributors loose about 36% on their royalty rate, diamond contributors loose about 53%. The more of your files are available at other agencies, the more likely they are to make up for this difference.
The obvious choice when deciding for a new agency is shutterstock and pond5. Other agencies, that offer smaller revenue, may not be worth the trouble of uploading and managing your entire portfolio. Other than that, ignore agencies paying less than 25$ for licensing a high definition file. Trying to gain market position by selling your footage for cheap hurts our market. Please do not undervalue your work. Apart from istockphoto I choose to work with shutterstock, pond5 and revostock. Clipcanvas is digesting my upload for about two months now.
When creating accounts on other agencies, be sure to use referral links. You can use mine by going through the banners on my blog, or anyone else you feel like supporting for helping out the community. There are many artists out there who deserve it.
Before you do that, even before you cancel your exclusivity contract, prepare your portfolio. This however requires a separate blog post