Matt Moore wrote a great article about how you can improve quality of your rails code called Ruby on Rails Code Quality Checklist .
In my opinion it is a very useful article for especially rails beginers.

Here is the checklist

• Each controller action only calls one model method other than an initial find or new.
  (Make custom .new or .update methods in the model with all necessary).
• Only one or two instance variables are shared between each controller and view.
• All model and variable names are both immediately obvious (to a new developer) and as short as possible without using abbreviations.
• All custom “finds” accessed from more than one place in the code use named_scope instead of a custom method.
• A .find or .find_by_ is never called in a view or view helper.
• There is zero custom code that duplicates functionality of a built-in function in rails.
• Code has been aggressively DRYed during development.
• All functionality used in two or more models has been turned into a library/module.
• All logic duplicated between two or more apps has been turned into a gemified plugin.
• STI is not used anywhere
• Every design choice should yield the most simplistic design possible for the need of users at the current time.
  No guesses for future functionality were designed into the application.
• Close to full test coverage exists at the highest level of the application: on and between controller actions.
  Coverage is highest for code used by the most number of end users.
• All tests pass before code is merged into a shared repository.
• Every fixed defect on a deployed product has tests added to prevent regression.
• Every plugin installed has been code reviewed.

He also explains all the items in the checklist in a very clear way in his blog so I strongly suggest you read that article

Another good article about this topic: http://www.fortytwo.gr/blog/18/9-Essential-Rails-Tips