Code Contracts - a simple idiom

I've been experimenting with .NET 4 Code Contracts for a couple of months.
Contracts themselves are simple enough to understand. The potential to produce stabler code, improve readability, and some help with the pesky 'object null' problem makes them very attractive.
A few blog posts, such as Billy McCafferty, and a series from Derik Whittaker help get up to speed.

My initial work with Contracts started with simple require not null type contracts, and soon escalated to more useful contracts such as 'no nulls in a list', FileInfo.Exists == true. What I found was a simple rule helped to limit what to declare for a contract:

Don't Require more than what a caller can simply (one-line) Ensure.

It is easy for a caller to check an object is not-null before calling a library... but out-of-the-box checking for more complex constraints, such as list contains no-nulls made the code less readable. The idiom above doesn't stop me from using more complex constraints, but it does ensure I provide a simple (one line) way for callers to ensure the constraint.