I'm moving onto the level design of this short game I'm deving and I came up with this little chart to help with the difficulty curve. I'm doing this with graph paper, but you can make an excel document or whatever. Once I designed all my obstacles(these are example obstacles), I arranged them according to their difficulty. Then I made 3 phases( you can add however many you want) for each level. This gives each level a flow. Have you ever listened to a song that is at the same intensity all the way through? There are some exceptions, but generally it helps a song if it has some dynamics. The same goes for level design.
It can be tweeked in many ways to fit different genres. For example, if you are making an exploration game, you can replace level x with the name of a zone: "Highlands".
I just wanted to throw this idea out there to all those visual people. It's helped me with my current project.
Take care.
Developer
