A tough situation I think some people face with the most basic implementation of scrum is that their backlog items are too big. Well this backlog item is "Foo" functionality and it delivers no business value without the whole thing so it's all or nothing.

Is this happening in your organization?

When we go into sprint planning, we have the opportunity to estimate that backlog item again, mentioning how big it is. Hypothetically let's say your backlog item is sized (relative point size) as 150 and you know your team velocity is 150 points. This means that you will onlly be able to do that 1 backlog item of 150 points that whole sprint.

So what happens?

Someone says: Surely you could do more than just that one thing! You got more people than could possibly work on that item!

Well, yes and no... we have more people than could possibly work on that item, however, we have more item than we could possibly work on with this much iteration length as well!

So one answer that's out there, which is proven and tested in the field, is user stories.

Mike Cohn describes user stories in a couple different flavors... the most basic and small sized stories are just called user stories, but when you have something as large as a whole iteration, you're probably dealing with what he calls an epic! Maybe it's bigger than an Epic and it's a whole theme! (i may be getting theme and epic mixed up but the point remains the same)

An epic is a collection of functionality that represents one functional whole to the user. Take this Epic for example:

As a web bank user I would like to transfer money from my savings account to my credit card.

That seems innocuous enough right? Well, maybe... let's look more into it though. Can we identify smaller stories in there?

As web bank user I cannot transfer more than my available balance from my savings account to my credit card.

as a web bank user i cannot transfer less than 1 dollar from my savings account to my credit card.

As a web bank user i can do recurring transfers from my savings account to my credit card.

As a web bank user i can do scheduled transfers from my savings account to my credit card.

As a web bank user i can do a minimum balance payment from my savings account to my credit card.

-=== and so on.

And just for the sake of mentioning... a theme would be something like

As a web bank user, i want to transfer funds.

as a web bank user, i want to pay bills online