I'd suggest the following:
Join up active gaming forums and watch discussions on E3 expos and events. Some one will tell you if tickets are available. Also sign up on some ticketing sites and ask for updates on E3.
You can't buy tickets, but you can get one if you have buying for any company that sells video games. Go to the E3 website to sign up. You'll have to submit proof of your employment (a W2 form, for example).
I think the last Public Day event (E3 expo) got over in June 2009. I don't know when the next one is. Hunt around to find when the next public day - they usually have a website up and you'll get info there.
Check out the official e3 website (e3 and the e3 summit website). They will have official info on when public tickets will be available and how to get them. Unfortunately they are not easy, but if you're a die hard fan you will find some available!
The way to get tickets to E3 is to be in the industry. You can only get in if you have an invite but some years they have a public day. If you do not work in the industry by working for a gaming company or on the business/computer end you may try seeing if they have a public day.