UI guidelines can be helpful to provide rules-of-thumb, or at least remind you of things to think about when designing a UI. What sets of guidelines exist?
This list is pretty comprehensive: The UX Bookmark: A huge list of Style Guides and UI Guidelines
An updated list available at UI Guideline Resources
I would look for UI design patterns. Design patterns are proven solutions to common problems - they started out in the field of architecture but have been ported over to design and development. For an introduction into design patterns, you could read A Pattern Language and The Timeless Way of Building, but both of these are concerned with architecture and not design/development.
It's vital to outline 'what' you're designing. There are certainly design patterns and guidelines for a variety of platforms but also for a variety of audiences. If you're talking about designing for the web Yahoo has one of the most comprehensive and open collections of patterns out there: (http://developer.yahoo.com/ypatterns/) otherwise it really depends on what you're trying to design. You're best served searching for patterns and going from there - the Wroblewski form design book is a great example of drilling down to specific tasks/elements for insight.
While it's not exactly a "Guidelines" website, I like to think of it more like a "How Not To Do Things" website. It's hilarious at times because most of it is so true.
There are many guidelines out there, coming under different banners too:
- best practices, heuristics (notably the ones from Jakob Nielsen: http://www.nngroup.com/articles/ten-usability-heuristics/)
- design patterns libraries (such as the Wellie ones: http://www.welie.com/patterns/) and books (Design of Sites by Douglas Van Duyne et al: http://www.amazon.de/The-Design-Sites-Patterns-Creating/dp/0131345559), and so on as already mentioned.
You might want to filter the lot as the applicability depends on what kind of UIs you are designing, the context of use, enabling technology and so on. For example, the Oracle Fusion Middleware enabled Fusion UX Design Patterns and Guidelines (disclosure: I work for Oracle and know about these) for enterprise apps: http://www.oracle.com/technetwork/topics/ux/applications/gps-1601227.html
A good place to start IMO is with an overall philosophy of where you want to go with your UI and see what support there is for bringing those thoughts to life as artfully as possible. Try:
- 37 Signals: Getting Real: http://gettingreal.37signals.com/toc.php
- A great jumping off point and one you can come back to is Steve Krug's Dont Make Me Think: http://www.amazon.com/Dont-Make-Me-Think-Usability/dp/0321344758