I found two gmail tips on Everyday Loopholes. I think they're pretty clever. I found another on Google's website.
One is that you can append things to your gmail address. An email addressed to firstname.lastname@example.org will reach the valid google email address. Here's the link to "Gmail Address + Anything You Want" at Everyday Loopholes.
The other is called "Google Twofer" on Everyday Loopholes. It points out that signing up for gmail actually means an email address that ends in gmail.com and another that ends in googlemail.com. Both go to the same mailbox.
Lastly, dots are meaningless in Google address names so all of these emails go to the same address:
email@example.com = firstname.lastname@example.org
email@example.com = HOMERJSIMPSON@gmail.com
firstname.lastname@example.org = Homer.J.Simpson@gmail.com
Google explains it here.
You can see the results of test messages by clicking on the "show details" link in the Google mail message. As Everyday Loopholes suggests, it might be useful to use some of these alternatives when signing up for email at websites and then filtering if necessary.