Dual Screens
I put my editor on one screen, with Safari on the other. Flicking between them is much faster. Also it's great to have my database (mysql in Terminal), designs I'm working from (Freehand) and the resultant page (Safari) in view while I'm typing code.
Use click-through for faster developing
Switching between text editor and web browser is often repeated many times when developing a web page. I use "click-through" to refresh my web browser page, without bring Safari to the front. Hold Command and use the mouse to click on the refresh icon. The advantage is the caret remains active and in place in my editor and there is no delay bringing windows to the front.
Assign "Command Click" to a button on mouse
I have attached "Command click" to my scroll wheel button on my mouse for several benefits. The "click through" mentioned above is even easier. A middle click on a link will open it in new tab. Moving, resizing and even scrolling windows around behind other windows is handy.
Leave Cookies view open
When developing Cookies, I have found Safari will let you leave the cookies panel open while working. Find Safari->Preferences->Security and choose "Show Cookies". I normally clear out all cookies before starting so I can see the ones I am working on. This panel is dynamically refreshed, so its so easy to see if cookies are being set or not, and what to.
I hope this gives other developers ideas.
robg update 10/27: At the author's request, I changed the original references to "option clicks" to the correct "command clicks."

