Number One tip: stop quitting your most-used applications! I see scores of people use their PCs. Almost everyone will do the following:
Open their browser. Quit (close) their browser.
Open their email program. Quit their email program.
Open Word. Quit Word.
Then they repeat this process. Over and over.
STOP. Here’s the tip: keep your most-used apps open all the time. NEVER quit them. This includes your browser (Firefox, Safari), your email app (Mail, Entourage), TextWrangler, etc.
You will run out of contiguous memory by constantly opening and closing apps. I normally have all of these open, and never quit them:
Firefox. Mail, TextWrangler, QuickTime, Preview, iCal, iChat. Even Dreamweaver.
Keep ’em open!