Choose a design that suits your blog topic/theme
This is less a case of common sense and more a case of picking something that your viewers are actually going to like. The theme or topic of your blog is probably going to have some sort of theme already attached to it. For example, a blog about the history of the tulip is hardly going to have a design that is blood soaked with images of severed limb. Even though this is an extreme example, it does show how not knowing the target audience is going to affect your blog design choice.
Pick a design and then customize it to your tastes
This is probably the best option you have. You should pick a design that is tried and tested, but then customize it to your tastes. Sometimes just a few changes can make a big difference to the tone and flavor of a blog design.
Sometimes a lack of color is better
This is a tip that it seems to take people a long time to learn. Color looks great, and that is part of the problem. People come up with some really great blog designs that are full of color, and they look good, but they draw too much attention. The text is the thing that people are coming to your site to see, not the flashy colors.
Mock up a few blog designs and compare them
This is probably the best method for choosing your blog design. Some designs look great on their own, even if they are not very good. If you stack up a few side by side you will begin to see the flaws in each one.
Put practicality above design
You must do this, even if it makes your design look bland and lack luster. You need put practicality first every time, because people will not return for your design, they will return for the use that your blog gives them. Therefore if usability is hindered in any way, you are going to have trouble getting people to re-visit your blog.
A fixed header bar
This is a common blog design, and is handy depending upon how your blog navigation is set up. If the user is expected to do a fair amount of scrolling, then this sort of thing is going to help him or her. However, make sure your header has some useful links on it, otherwise it is pointless.
A big mix of font styles and sizes
This design is still commonly seen and does seem to work for some blog posts. The only mistake you should avoid is putting a lot of different colors on the site. The affect of the different font styles and sizes is not often perceivable straight away; it often just gives the blog texture. If you add color then you throw the dynamic of the page and it often ends up looking a mess.
The magazine or book style design
This is becoming more popular as a blog design for the simple reason that it is different from other blogs. There are also people who are making blogs with Flash, and using this design, whereby the user can flip a page to see the next blog post.
An overly complex blog layout
Do not misunderstand this design (as many do). It is more about organized chaos than it is about convolution. Make your blog complex in design, but intuitive in use. Do not make navigation any harder than it has to be. Think of your blog as a buffet full of food. Your buffet table will look full and complex, but it will not insist that people pick up their food with a rusty nail.
Using lots of widgets in your blog
This is popular because it is increasingly easy and effective. However, be careful not to dim the focus of your text in your quest for more widgets.
Minimalism with lots of white space
This is still a popular design, you just have to make sure that the text you add is not too small (in font size) and make sure that your paragraph size is not too big, as this will scare off most readers.
Large photo backgrounds or templates
This is a website design that is gently seeping into the blogging world. If your blog uses caching software then you could do far worse than installing this type of blog design.