I've found that following the following will lead to an easier time when posting your item.
1. ALWAYS obtain proof of postage. This is generally only a slip of paper the postoffice fills in, but it does prove to the buyer that you did send the item. (even for letters)
2. Try and offer recorded delivery of your item, its just less than £1 extra (currently 68p) and worth doing as it gives the buyer peace of mind as they can track the item. It will require filling in a small form with addresses, but since the customer will be paying for postage it wont cost you any more.
3. Printout the item description and include it with your item. If the customer has bought several items from different people it will be easier to give you the appropriate positive feedback.
4. Email the customer when you've recieved payments such as cheques and tell them when you posted the item and include any reasons for delays. Keeping other people updated means better feedback for you.
5. Obtain packaging before selling your item as this can speed up posting and leaves you more time to deal with other sales.
6. While your sorting out your packaging get your item weighed and find out how much postage will be. Generally I don't ship abroad, but at least you can look up the cost on the postoffice website if you already know the weight.
7. I work 2 minutes away from a postoffice so can offer to send the item as soon as payment has cleared. Inform your buyers how soon you can post items as they may want to get them quickly.
8. Many people offer cheap items with high postage costs to make it cheaper to list the items. Since ebay is now giving buyers a chance to comment on these costs it's better to keep the postage costs low and get better ratings.
If you can suggest some more tips then email me and I'll add them too.