Read the appropriate site guidelines. There's no point in spending your hard earned and then discovering that you do have to disclose the URL. And that you can't have your sale also listed elsewhere like eBay.
Prepare properly for the listing, get all your ducks in a row. Collect the information that buyers typically ask for e.g., traffic and earning stats and have that all to hand. Create some graphs and images that demonstrate the key selling points, people like pictures. Store these, and any spreadsheets you create, on your own site if you wish for control over them or allow for future deletion. Do the type of due diligence checks your buyers are going to be doing so you can uncover any adverse signals and explain them or compensate for them.
Think carefully about your title. This is very, very important. The title is your bait to draw bidders in. Don't be tempted to use it to boast about your achievements. Draw up a shortlist of the top reasons someone would want to buy your site. Then trim it to just one or two. And draft and redraft those to convey the message in the most succinct and inviting way. Don't lie, don't exaggerate, don't use all caps.
Some recent examples of bad text in titles:
Made $75 in 2 days (draws people's attention to your lack of history and gives them an excuse not to click)
mywebsitename.com is for sale (unless you're a well known name this is likely to be meaningless to the average buyer)
BARGAIN - DO NOT MISS (comes across as immature. Don't use all caps and don't beg for buyers to read your auction.)
abcdmysite.com: A site dedicated to the World Reknowned Game (this isn't a speech at the Oscars, no dedications are necessary. The title is not the place for extended explanations of what the site is about)
In the auction listing itself, provide as much information as necessary without overwhelming buyers. Explain clearly what your site does and how it makes money, don't assume everybody will figure it out. Talk about your traffic and where it comes from. Be honest. Don't boast about fake PR as someone will catch you out. Don't claim that all traffic was free if you've been running paid ad campaigns (as even that can be uncovered in due diligence).
This isn't a stage for your own talents. Boasting about how many Digg frontpages you get or how clever you are at SEO may be counter productive as that suggests to the buyer that you are key to the site's success.
Manage your comments. Yes, your auction comments need to be managed. Show good faith by amending your auction listing if something comes to light that you didn't disclose earlier - or add a comment. Delete comments that add no value .e.g, "fantastic site, good luck with the sale". Answer questions publicly where possible as it will be of use to buyers watching from the shadows.
Don't change WHOIS once your auction has started - leave that to the winning bidder to do on completion of sale. Don't make any major site changes in the interim either or you'll spook some buyers.
Appreciate that sometimes buyers pull out post-sale so keep non-winning bidders on-side, don't burn your bridges. You may need to get back to them with a second chance offer.
For goodness sake, use an escrow service if the value is high enough to lose you some sleep should you get conned.
There are some more tips I've written about elsewhere but please feel free to add your own below.