1) Sending in just penny sleeves with no further protection: This is a no brainer but is something that happens on a more common basis than I like to see. Why would anyone think that this is acceptable? The mail is thrown, crushed, and just about any other thing you can think of, so why would you send the cards like that. It's disrespectful to the people you are sending to and to the hobby in general and is just lazy. Please use protection.
2) Tape: This is an area where balance is key. No tape or too little tape can lead to cards slipping out of the protected area and into danger. Also, too much tape is annoying. How many times have you received a package that you have been waiting for for weeks and then you got it only to spend a hour fumbling with tape?! Save the money on the tape and put just enough to hold in what you need. Also, it takes 2 seconds to fold the edges down so the receiver can just pull up the edge and not have to spend time and fingernails trying to get it up. It also saves money on the toploaders and things that have a trillion tape marks on them. We don't need the companies taking any more money from us.
3) Penny sleeves and cardboard/toploaders: If you use cardboard or toploaders to sandwich cards only in penny sleeves, please use discretion. This is an extremely common practice that when done correctly works very well, but when not done well will damage all the cards. Tape should be put on very securely so the cards don't slide. The edges and corners wear when not held in place. Also, watch that the cards don't slip toward the top of the penny sleeves and get tape juice all over them. If you squeeze the toploaders/cardboard down tight and tape tight, then the cards will stay in place.
There's a bunch of other suggestions I can think of, but I'll leave it open to you guys for your thoughts. Good luck and happy shipping.

