I've been updating the documentation for S#arp Architecture and thought I'd share the tips I'm including for NHibernate.  (When referenced below, "NRD" stands for NHibernate Reference Documentation.)

General Mapping File Tips

• For organization purposes, only declare one class per mapping file.
• For consistency and better organization, regardless of using HBMs or Fluent NHibernate, keep the mapping artifacts in your data layer project, e.g., MyProject.Data.
• Don’t bother including database meta data (e.g., column length) in mapping files unless intending to auto generate the SQL using the hbm2dll tool.
• If using HBMs, set the assembly and namespace for the <hibernate-mapping> tag to decrease typing and ease future maintenance.

Associations

• Set .Inverse(), or inverse=true if you’re using HBMs, on the parent in a bi-directional parent/child relationship to avoid foreign key constraint violations while making the relationship easy to maintain (NRD 6.8 Bidirectional Associations).  But note that large bags mapped with this setting are inefficient and should be avoided (NRD 6.2 Mapping a Collection).
• When mapping many-to-many relationships, use an <idbag> for much increased performance (NRD 6.7 Using an <idbag>).
• Remember that there is a difference between <bag> and <list> (list having an index property) and should be used appropriately; either may be mapped to a generic IList<> (NRD 6.2 Mapping a Collection).

Mapping Strategies

• While there are a few inheritance-mapping strategies to choose from, favor the table-per-class-hierarchy mapping strategy but consider your needs carefully, as well (NRD 5.1.13 subclass).
• If you’re dealing with money, create a Money class and an associated ICompositeUserType for the monetary amount to bind to it, e.g., here.  Storing money as a decimal in your domain layer is quite limiting.

This list isn't comprehensive by any means, but hopefully you'll find a couple of useful tidbits.

Billy McCafferty