Getting your domain name in front of buyers is obviously critical.
Two main approaches:
1. Someone else sells it for you Afternic.com and Sedo.com are the two biggest "domain listing" sites. These are great places to get your name in front of many domain investors. For very good domains, Afternic has sales people that will help find buyers.
If it's a very valuable domain, you may also want to contact a domain broker, like Rick Latona, who will help find the best buyer (for a cut, of course). They have experience selling the very best domains PLUS an extensive customer of bigtime investors to draw upon. If the domain isn't quite premium, the broker services will still work, but you may have to wait a little longer.
2. Sell It Yourself The second approach is selling it yourself -- this requires a lot more work on your part, but generally can lead to much better results. Selling directly to an end user will most often get you a better price than selling to a domain investor or an auction.
Think real hard about what kind of business would want your keyword domain and contact them directly. Many online businesses are very savvy about the value of a great domain, and will be open to your contact.
If you can't think of a company to sell to, try Googling for keyword in your domain name. See which companies are advertising for your keyword (look at the ads on the right after you search). These companies are already PAYING to advertise for the search term, and so they will see the value of your domain.
Another tip is to search Google for "INURL:keyword". This will give you a list of companies that may be using a longer version of your domain like "MyKeyword.com", when you own "Keyword.com". These companies may be very interested in the "better" domain that you have.
I'll also plug my iPhone app -- search the app store for "Domains For Sale ", and you can sign up for a free domain listing and sell it yourself.
META: Maybe the right place for this question would be an SE site like AskAboutDomains.COM. It's too hard to build traffic when the questions go to the wrong site :) [good natured jab]