Whether you're new to web design or well past the beginner stage, these tips from David Kadavy can help you simplify your slides, web page or whatever it is you're making. The Design for Hackers author's nine tips cover everything from choosing fonts and colours to keeping an eye on white space.
This one seems especially useful as a rule of thumb:
With digital typography, we have the unfortunate luxury of being able to make type whatever size we want. I say "unfortunate" because for most people this is a detriment as it complicates things. Do yourself a favour, and only use these target pixel sizes: 5, 7, 9, 12, 16, 21, 28, 37, 50, 67, 83. These sizes are based off of the proportion of 3:4, and you can read more on the technical details of arriving at these sizes in How to Establish a Modular Typographic Scale.
If one of these sizes isn't quite achieving the effect you want, there are tons of other things you can do: use bold or italics, use all-caps (for titling and metadata), or use a different colour. Stick with these sizes, and worry about something else.
There's an endless sea of options you can choose when designing something, but Kadavy's tips help make design less complex. By the way, his free email course, Design for Hackers, is open again for registration.