Here are some of these tips that will make your experience with the Desire more enjoyable.
The often use of the optical track-pad can make your life easier when browsing the Internet, sending an email or simply editing a text.
Even though many people think that the track-pad is useless and that HTC could've made the screen larger instead of including this item, I still think it saves a lot of time when it's used for specific tasks.
For example you can easily use it for precise cursor placement when you're editing a text. Also, it's perfect for going back to correct a block of text.
To select a certain text in text editing, just place the cursor where you want to start your selection and long-press on the optical track-pad to bring up the context menu.
Then tap “Select text” and use the optical track-pad to fine tune your selection. Further, you will then be able to cut or copy the selection, by long-pressing on the optical track-pad to bring up the context menu and choosing one of the actions.
While in the browser you can use the track-pad to jump between links, which is very useful in case the links are very small and hard to pinpoint with your finger.
There's also an interesting feature that enables you to open the link in a new window, when you long-press on the optical track-pad and choose the action you need from the context menu that it displays.
Feel free to add more of these tips, especially if you're an advanced Android user.