Here is what I usually do:

 1. **Always develop with `error_reporting` on.**
 2. **Always develop with `isDeveloperMode` set to true. Just add `SetEnv MAGE_IS_DEVELOPER_MODE 1` to your `httpd.conf` file (or corresponding file for nginx or something else)**
 3. If the extension is linked to a core functionality add the
    dependency in the declaration file `<depends><Mage_Catalog /></depend>`
 4. If the module is for community use, use `community` as codepool to
    give the developers the chance to override some classes without
    modifying the code directly
 5. Put your frontend design files in `app/design/frontend/base/default`
    to make them available for all themes
 6. Put your admin design files in
    `app/design/adminhtml/default/default` and do not change the admin
    theme. I may want to change it in one of my modules
 7. Prefix your layout file names and template folder name with the
    company name to make it easier to isolate them
    `easylife_articles.xml` and `app/design/.../easylife_articles`
 8. Put your static resources (js, css, images) in a similar folder as
    the template files `easylife_articles/images/doh.png`
 9. Attach a simple text file with how to uninstall the extension: What
    files need to be removed, what tables need to be dropped, what
    config settings need to be removed from `core_config_data` table
 10. Do not write queries directly in models, blocks or helpers, use a
     resource model for that
 11. Do not write queries using the table names directly `Select * from
     sales_flat_order where ...`. Use a `Zend_Select` and transform the
     table names using `->getTable('sales/order')`.
 12. Use the base url to include `js` files in template. **Wrong**
     `<script type="text/javascript" src="../js/some.js"></script>`.
     **Right** `<script type="text/javascript" src="<?php echo Mage::getBaseUrl('js').'some.js'?>"></script>`
 13. Do not rewrite classes unless is necessary. Use observers and if
     it's not possible use helper methods that receive as parameter and
     instance of a class that you wanted to override. **Wrong**:
     Override `Mage_Catalog_Model_Product` to add the method
     `getProductActicles()`. **Right**. In your helper add
     `getProductArticles(Mage_Catalog_Model_Product $product)`
 14. If you override classes put a list of them in a `readme.txt` file
 15. Use the default admin path for the admin section of your module.
     **Wrong admin url** `articles/adminhtml_articles/index`. **Right admin url** `admin/articles/index`
 16. Add ACL for your admin sections. I may want to restrict access to
     some of the admins.
 17. Do not add an other js framework (jquery, mootools, ...) if it's not
     necessary. write you code in prototype.
 18. Make you template html W3C valid (this is for OCD developers like myself).
 19. Do not put images in the `media` folder. Use `skin`. The `media`
     folder usually is not versioned and this make it harder to move the
     website on different environments.
 20. Test you extension with flat catalog on and off. In order not to double the dev time use [Chaos Monkey][1]
 21. Test your extension with cache on an off
 22. Avoid using uppercase letter in the module and class names. If not
     properly tested this may cause issues on different OS. This is more a recommendation, not a 'must'.
 23. Dispatch events in your code to make it easier for developers to
     alter the functionality
 24. Follow the same coding standards that Magento uses and comment your code.
 25. *[Edited]* Do not use php short tags (`<? $this->doSomething() ?>`). Use full tags (`<?php $this->doSomething()?>`). Also don't use short echo tags, yet. (`<?="D'oh";?>`). Use (`<?php echo "D'oh";?>`)
 26. Translate your texts using `$this->__` and add the locale translation file with your texts (`app/local/en_US/Easylife_Articles.csv`) at least for `en_US` language. Not all
     websites are build in English and the identification of texts to
     translate it's time consuming.
 27. If you sell an extension offer at least basic support. Or at least
     answer the support e-mails you receive.
 28. **Do not make constant calls to your servers through your extension for licence validation. Once, at installation is more than enough (I don't like this approach either, but it's better than to make calls all the time).
     (Inspired by [this question][2])**
 29. Develop with the log activated and from time to time take a look at
     the `var/log/system.log` file. The errors listed here are not shown
     even with developer mode on. If there is at least one error you end
     up with a large log file after a few months of running the extension.
 30. If your extension affects the checkout process or the orders in
     some way, make sure it works with multi-shipping, or if it
     shouldn't work with multi-shipping make sure it doesn't affect it.
 31. **Do not replace the default Admin Notification bar (or feed url). If
     I'm interested on what you have to offer I will subscribe to your
     newsletter. Let me see what magento has to say. It's more important
     to me.**
 32. **If you encrypt your code files with ioncube (or something
     else)...well...I just hate you and I hope your business goes bankrupt**

That's what  have so far. I will add more as soon as I thing of something else.


  [1]: http://alanstorm.com/magento_flat_collection_chaos
  [2]: http://magento.stackexchange.com/q/8460/146