How to write a custom extension?

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_datatable.
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 makes 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 development time useChaos Monkey
21. Test your extension with cache on and cache 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)
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