How to make program, suitable for selling as shareware

1. You must have idea about program category and do some market research. Go on some sites that are selling shareware and freeware (www.download.com for example) and find out how many similar programs are offered on these sites. Try download some of them and think, if you are able to write program with similar functionality, but more better. You program must have some unique features that gives you more chances on software market.
2. Think about your potential market size. How many people could use your program. It must be big part of population, otherwise you will not be able to sell your program.
3. Think about system of registration from trial version to full version. And also about limitations for trial version. There exists some standard packages providing this.
4. Your program must be user friendly. Don't force possible customers to do something manually outside of your program. Use standard input controls for your GUI. Good advice is give your program for testing somebody who is not related with programming. When you are testing DIY (do it yourself), you will never find how much bugs do you have in your program.
5. No bugs in your program (Especially in trial version).
6. Use a lot of reports (the best of all in graphics) in your program (when it is suitable). Sometimes program with good design has higher rank than program with more functionality.

There are more rules, but I think that rules above are the most important.