J.C.
I have the dreaded wavy hair too! But, I've actually come to realize that I am the only one out of all my friends who can straight OR curly hair, depending on how I style it. It took a long time for me to come to this realization and embrace it, but I figured "well, I'm stuck with it, so might as well find the positives". So, here are some suggestions..
If you want it straight, you are going to have to blow dry it. There's a product by Big Sexy Hair called "Straight Sexy Hair, Smooth and Seal" It is the ONLY product out of probably 20-30 things I have tried throughout the years that gets my hair stick straight with no frizzies. When you use it, pull all of your hair up into a clip except for a fairly small section going all the way around the bottom of your hair in the back. Spray and flat iron. Then undo the clip while holding onto your hair and use your finger to part out another section. If you do it in thin sections, you'll get the best effect. I used to do it in about 4 different sections, but then was told to use smaller ones, so now I use about 10 and LOVE the difference!
For curly hair, you have the right idea, mousse and diffuse.
If you want to let it dry naturally, the only thing I've come to know that I can do without having nappy hair when it's dried is to put mousse in it while it's wet. It dries with a lot of wave, but the mousse keeps it from getting frizzy, so it's not bad at all. Then, I am a bobby pin junkie. While it's drying, I usually twist my bangs back and bobbie pin them, totally cute!
Oh! If your hair gets crunchy to where when you shake it out with your hands it doesn't soften up, use a little less mousse until you find the right amount. When I wear my hair curly though, it's usually a little crunchy.