Lately I have been experimenting with braids and I love how simple this technique is, yet there are still so many different ways to wear them. First of all, braids do not always have to look juvenile (unless you are doing french braid pig tails, then, well, it's going to look childish). Try to do a sideways french braid, either pinning the pony tail part under, creating a side bun or just letting it be. One of the best tips I got from another stylist was "There aren't any rules. If you like it, let it be." So if it looks messy and you like it, wear it!
I found this great video that will show you how to do a variation of a french braid on yourself that can help you get some ideas and looks nice. With practice it will become easier, I promise!
Here are some more styles for inspiration, including videos on how to do a fishtail braid and a rope braid, which aren't very common but still something I love to see.