For plastic windows in envelopes, and magazines in plastic:
I've finally given in and signed up for paper-free billing on telephone, gas, electricity and internet. I don't really like doing this as I think I'll forget to check them but I'm paying them all by direct debit so I suppose it doesn't matter too much.
In the process of switching to internet banking and online billing - coping with some paranoia about fraud, which is terribly backward of me.
I also took the advice posted in Andrew's earlier comment on this blog: returning incorrectly addressed mail to the sender, opting out of unaddressed mail with Royal Mail, and signing up to the Mail Preference Service to stop getting unsolicited (junk) mail. This doesn't stop you getting things like your local papers or election information from the council or political parties, etc, so we will still probably receive some plastic in the post, but some is better than lots!
On a related note, I've started making cards for birthdays and other occasions a lot more than I was. You can still get postcards and some cards that don't come in plastic, but a home made card's much better! The card industry is SUCH a rip-off!