1) Try SomeModel.make_unsaved if you don't actually need it to be saved.
2) Look for ways to create lighter versions of the objects. Maybe you can avoid creating the associated objects. For example:
BlogComment.make(:gravatar => nil) # avoid creating the gravatar image
Or if you usually need the lighter version, you could flip that around and have the regular BlogComment.make create the light object (no gravatar) whereas BlogComment.make(:with_gravatar) would do the extra work.
3) Avoid offsite hits to web services (Google maps, S3, etc.) if you're not specifically doing an integration test for that service. Use stubbing to prevent those methods on the object you're testing from actually connecting to the external service.