Language learning software

[pogo]

I'm thinking of writing my own language learning software (Spanish will be first) but I'm a bit unsure of the best approach to take to create something that will actually help people learn.
At the moment I'm thinking of having a number of lessons with each lessons revolving around a simple conversation, the point of the lesson is to help you understand everything in the conversation. But beyond that I'm not sure.
So I'd love to hear any suggestions people have about what they've either found useful or think would be useful.