Most important, you need to know what you want your sequel to do. If the only reason to have a sequel is to bank on the audience buying it because of brand name recognition, that is an approach that will guarantee a small number of sales, while sacrificing customer goodwill in the longer run. Each succeeding sequel following that strategy will generate fewer sales, until the brand itself loses all cachet. The Army Men franchise fell victim to this phenomenon.

Conversely, the best reason to have a sequel is because you have more to offer. Say you want to iterate on an interesting gameplay mechanic, or you've discovered the roots of a deeper storyline from the bones of the previous one -- any time that you feel you have more to give the audience, then a sequel is a good idea.

So once you've decided you want a sequel, what are the key elements for success? First, don't make radical changes. Chances are that people who loved your first game love it for very specific reasons. Figure out what those reasons are, and keep them for the sequel. If it's gameplay, keep the same gameplay in place, and iterate upon it. Improve it if you can. If it's the plot or the characters, work on that, but don't lose sight of what made those things great. Give the audience more of what they enjoyed from the first game.

Second, listen to user feedback. In these days of easily-enabled communication, players can take to a game's website's forums to praise and complain about all manner of things. It's worth the time it takes to wade through what the players are saying to see if there's meaningful content there. Chances are there will be something you can use to improve the sequel. Sometimes it's something to add, sometimes it's something to remove. The trap, of course, is that there's usually so much feedback that it can be difficult to winnow it down to quality. But given that these people are the passionate ones who will be most interested in your sequel, learning about what they loved and hated is invaluable.