TIMING... The start and finish time of an auction is critical to getting the bids in. Theres no point spending 45 minutes designing the most beautiful listing for your item if you then set it to finish at 4 in the morning! most people will bid within the last few minutes of an auction and because there are so many items listed on eBay people will rarely scroll through more than a few hours worth.
By setting your auction to finish at a time more suited to the buyer you will get more bids on your listing. (i.e. If your selling to businesses ensure your listings don't finish over lunch time or after about 4pm, Whilst if your selling collectables you should aim to end the auctions around 7 - 8 pm when most people are at home and logged on).
Consider things such as public holidays, sporting events, TV prgroammes, and even major film releases. I once lost a huge amount of money by inadvertently ending some auctions during the first England game in the world cup!
Another thing worth remembering is when people get paid. Generally people have more to spend at the beginning of the month than they do at the end. Use this to your advantage by listing your more valuable items at the beginning of the month.
Use seasonal variations to your advantage as well. Nearer to Christmas people will be looking for gift items, at the start of the summer people will be looking for motorbikes and garden furniture. Expect to get more for your parasol in june than you will in december!