In no particular order! Please leave your ideas in the comments below.

1. LABELS Print them yourself:
~ with a laser printer with refillable and recyclable cartridges.
~ in small batches as needed so that when you change a formula or size or anything, you don't have to throw away labels
~ on post consumer recycled paper labels

2. ENVELOPES AND PACKAGING:
~ Reuse only. I post my contragels, for instance, in an envelope inside and envelope. These envelopes are ones that come in the mail, especially those envelopes enclosed by utility companies and charities.
~ Question the use of padded envelopes. Double enveloping may do.
~ Use paper tape. It's all I use, even for labelling packages.
~ I use freecycling groups to give away my excess bubble wrap. If you need bubble wrap, check in with a local business that you think is likely getting goods in bubble wrap, or ask on a freecycling group.

3. REUSE PLASTIC
~ I give away small plastic bags that caps etc are wrapped in to a cosmetic company to send samples out in.
~ I use larger plastic bags that bottles etc come in to pack the bulk goods I offer.

SINCE I STARTED MY BUSINESS IN 2007, I HAVE NOT HAD TO BUY ONE SINGLE PLASTIC ITEM FOR PACKAGING OR PACKING.

4. REDUCE AND REUSE PAPER
~ Use used envelopes to make bank/credit union deposits in machines. I use windowed envelopes and don't worry about sealing them. I put my cheques signed in there as they come and write the amount on the front of the envelope. When I think I'll pass by the instant teller, I add up the sum and put it circled on the front of the envelope. That way when I have a trailer full of aggravated children,. I can make a deposit in less than a minute.
~ Make all order notes on the order slip itself. All in one place - no staples, no tape, no clips, no losing stuff. I write the postage total on the order form beside the postage paid, and the order # on the postage receipt. After I enter these in to my lovely comprehensive spreadsheet, I can file them separately, yet reunite them if a parcel is lost or late to make a claim.
~ I email clients their invoices and receipts.
~ I leave reused envelopes for clients to put their money in when they are picking up orders and I won't be there.
~ Almost everything I print is on good-one-side reused paper - even posters!