Today is the deadline for nominations to the May 2015 IndieNext list.
The ABA makes nominating titles easy. Use this form on their website or click here to send an email. They recommend that you write conversationally, as if you were handselling to a customer. (And they even promise to take care of grammar and punctuation.)
If you’re looking beyond May, all of the deadlines for 2015 lists (and part of 2016) have been updated on the site here. So if you just finished a book that will not be released until later this summer, go ahead and nominate it. They’ll hold onto the nomination until the appropriate time.