Do you guys just dive right in after you get your initial idea? Or do you plan it out, right down requirements, wireframe etc.?
Maybe you even have some specific process if you have done a lot of projects?
Wireframes are pretty useful for making any web page, not just a hobby web app. My process is to sketch out wireframes, make the HTML and UI, then build the backend.
Also, I try to come up with at least three ways to implement a UI for a new app. That way you don't get emotionally tied to the first one just because you thought of it first. Usually, I find that my first one is crap, and the second one is good enough that I never can think of a third one that's better than the second.
As for design, for a tech guy, I think I'm pretty good, but one really nice thing I'm noticing is that web design seems to be getting simpler. Or at the very least, you don't get points taken away for being sparse. (Usability is different tho, I'll definitely take points away for that.)
One great example is stackoverflow. It represents a design I would have never made. I would have called it ugly. But I really like it for some reason. :)
Then I plan everything to the last detail, then I build it. When I was younger I just built it, and that's a great way to learn lots about both social economics and code. Nowadays I feel my time is better invested in producing something with a high chance of success than just a messing-about project. Unless the concept really grabs me, like http://thingsinbooks.com, in which case I just build it.
As long as you start somewhere, it doesn't matter, it is a hobby project after all.