For technical skills, I would recommend using Photoshop for web/ui design. Unfortunately at the moment it's the best out there. Some good things to seek out are adjustment layers and layer effects. Bjango has some great tutorials, although they are focused on UI design: http://bjango.com/articles/

In terms aesthetics, the most import thing is to develop your 'design palate'. Seek out excellent craftsmanship and learn to distinguish it from the mediocre. An excellent place to start is SiteInspire: http://siteinspire.com/

I would also recommend learning the fundamentals of design. Typography and grid systems are very important study areas. A great place to start is this forum thread: http://www.designerstalk.com/forums/graphic-design/42390-des...

For empirical testing of designs, I would highly recommend Lukas Mathis' Designed for Use: http://pragprog.com/book/lmuse/designed-for-use Be warned however that the print version is horridly designed, just get the .epub to bypass the ugly formatting.

And always have in the back of your mind: 'less is more'. Just because you've learned how to do fancy gradients, reflections and rounded corners, doesn't mean you should.