My 10 not so secret tips concerning mashup design

• Be smart! Today, Development is more integration than coding so always try to find the bricks (api, library, code,...) you need on the net and provide only the cement when possible. A quite good index of API is available at programmableweb.com.
• If you create one of those bricks for a non-commercial purpose, you should share it because, if you need it, you can be sure that someday, somewhere on Earth, someone will also need it to design something fresh which could inspire you (see the next section about geospatial data for example). Moreover don't think that Google or other search engines will index them correctly (at least at the beginning). Register them on suitable websites or forums. For google maps mashups, the Google Earth community forum is a good start.
• Create something you can use afterwards almost on a daily basis. By extension, create something usefull. A famous mashup like Flickrvision is very nice to see a few minutes but IMHO is without real added-value.
• Use an Integrated Development Environment (IDE) dedicated to web development, so with coding assistance for JavaScript, HTML, and CSS languages, an outliner of JavaScript, HTML, and CSS code structure, and error and warning notification. Personally I use the community edition of the Apatana Studio.
• Instal and use Firebug and IETab. Both are extensions for firefox so usefull to debug.
• Apply Yahoo! Performance best practices and instal YSlow for Firebug. You can also freely use online Pingdom Tools.
• Instal locally a packaged web server like XAMPP.
• For Cross-Domain XMLHttpRequest calls, use a proxy and deploy on a web hosting service supporting the cURL library. Be cautious and add a few php lines of code to prevent a hacker from proxying through it.
• Use a version control system like Subversion and log it on the mashup page to keep your users aware of releases.
• Give your users a mean to feedback on the very mashup page (email, form,..).