A few easy steps to looking busy at work when you're actually blogging/emailing/facebooking/playing spider solitaire/etc:
-If you use Windows, make sure you have enough programs running that your taskbar is full. This will squish everything together making it less obvious that you've got the internet or a game running.
-Open at least two windows of the internet so that when you minimize it it just says "Internet Explorer - 2" Instead of the name of whatever website you're at (like, say, The Receptionist), bringing more subtlety to the work.
-This is obvious, but make sure that at least one (but preferrably all, otherwise what's the point?) of the other programs you have running are legitimate work programs. Like a letter you should be editing, a spreadsheet you should be filling in with boring data (of course, have the papers with the actual data beside you as well, otherwise the illusion will be destroyed), or a fax you could be printing. That way when someone walks by you can quickly click onto the closest program and there you go! You're working.
-It can also be handy to have a pile of random papers that you can shuffle through as if you're trying to figure out what they are and where they need to be filed.
Things that could destroy this illusion:
-Slow reflexes. If you aren't able to sense someone's pending presence early enough they will see you do the switch-over, which is almost worse.
-Typing (or lack thereof). If the work-related "safe page" you've got is data entry and you're wasting time playing spider solitaire, it will be more obvious because of the staring and clicking vs. typing shift. Also if you're writing emails (or blogs) and then switching over to a page that would not involve typing, the abrupt stop to loud typing sounds is a bit of a dead giveaway.
-Progress. If you constantly go back to the same data-entry page whenever someone walks by you but you've never made any progress on it or just sit there and stare at it until they go by without actually doing anything, that may look fishy.
-Sudden action. When every time your boss walks out of his office he sees you spring to life and pick up the same piece of paper or suddenly start data-entering again, he may get suspicious of your work.
-Nervousness. If you get a jolt of nerves every time you have to hide what you're doing and sit up a little to straight or get that deer-in-the-office-caught-with-your-hand-in-the-blog look, you might be giving yourself away.
Of course, at the end of the day what's really going to destroy the illusion that you're getting work done is the fact that you're not and that it shouldn't take anyone an hour to change the address on a letter and print it out. Alas. Everyone knows you're not working, just accept it and move on.