.NET

• Use Caching
  
• When responding with an error, set the cacheability to no-cache! You don't want everyone seeing the same cached error message
• Don't use the IIS Development server. Use the local IIS as the development web server, so you can point the portal to your development machine and utilize debugging. This works for both web projects and websites
  
• If you use the same paths relative to the remote server, you won't have to change any settings besides the remote server when switching between development servers. For example, if your development portlet server is http://devserver/portlets/portlet1/portlet.aspx, (and the remote server points to http://devserver/portlets), make your local path /portlet1/portlet.aspx.
  
• Design everything to work OUTSIDE of the portal as well. This makes debugging very easy.
• I put <% if (isGatewayed) ..... %> inline in my web pages and then place the doctype, html, body, etc tags in there.
• All api calls / portal references should use dummy data or a service account for non-gatewayed access
• Avoid Complication (this will be controversial)
• App Accelerator. Yes, it saves you 4 lines of code. But it adds a layer of complexity that you will have to debug. Don't use it.
• .NET Web Controls. Yes, it makes portlet development very easy. Yes, it breaks lots of things. If you're good, you'll avoid it. More on this later.
  
• Don't gateway static resources
• Reference all static resources using <%= staticpath >/the/rest/of/the/path.html. then in the code behind, you can set the static path to be relative to the imageserver if its gatewayed.
• use the pt:common.include in head tag! This avoids the same javascript files being included twice, too!