I've been working on apps where packaged themes don't quite cut it due to UI requirements. I've never had technical trouble implementing ideas, but my layouts, colors, and general UI leave a lot to be desired. How do I improve my design know-how?
Gathering skill will take time, so for that read up on the sources often linked here. But to get your apps looking less than fugly asap, just copy others. Buy some cheap wordpress themes, view source, rip them apart to pick and choose individual elements. Copy the CSS from the best designed sites out there.
Initially it will feel like you're cheating, but over time you will learn the patterns and develop your own style.
EDIT: This couldn't have arrived at a better time - https://postgres.heroku.com. Just look at that beautiful design. A lot to be learnt (and copied!) there.
Agreed. It's very easy to learn by example, and with all the great CSS frameworks out there you won't need Photoshop to produce good-looking widgets. A lot of my early designs were based on ideas from this book:
There are a number of resources that can be found via Google (i.e. "app design") but Smashing Magazine has some good articles relating to app design and layout in general.
You can also try searching for "beautiful apps" which brings up links to collections of different apps that might provide an idea.
As far as color goes, I like searching for web site templates or 'best web design of 2011' collections and finding a color scheme that I like or think would work well with the general feel of what my product does. Remember, "Imitation is the sincerest form of flattery"
I used to stress about making my apps look good. Now I just use Skeleton (http://www.getskeleton.com/) to cover the basics and when my app is ready I pay a designer to make something that looks awesome.