Designing a WordPress theme is actually a tough job. I applause those who have created great WordPress theme. I think there is various ways you could consider to design your own theme or your could alter an existing wordpress theme.
Copy and Modify
This is a great way for me to learn CSS and WordPress code. Rather than reading through the long tutorial provided by WordPress.org, I kind of like to do trial and error myself. There are great theme from the official WordPress Theme directory, but there are also great theme out there that are not included in the Theme Directory because they choose not to release their theme under GPL. This means that you will have to read their license right and make sure you could modify and reuse their code, therefore be careful when you decides to remove their footer link.
This WordPress Generator is very cool because it require no coding at all. The downside is that there is limitation on some of the function. Buy hey, it is free and you only fill up the form. By 5 minutes, you will get your very own WordPress Theme. How cool is that?
Big Credit to this 24 year old from Rotterdam
PSD to CSS
I have not tried PSD to CSS much but it will be useful if you know nothing about CSS, but you have knowledge designing in PSD. This PSD to CSS is not design specifically for WordPress, therefore you will still need to make sure that you include all needed WordPress Code.
From a Blank Notepad
A shortcut from the above option
If you do not want to do it from a blank notepad, here is another tips to speed it up a little. Rather than typing the normal code for comment.php, searchform.php, header.php and function.php, now you can skip this process. This is the most common method because designer will have a standard blank templates to speed up their daily task. WPCandy have one Starter Theme that is very basic, without any width size or sidebars.
I have just recently made my theme (RiceBlogger theme) but it is not design by me. I have just added some sleek function to make the original default WordPress theme a better theme to match what is in the market today. The weird thing I have found out is that Kubrick Theme does not comply to the requirements. You will see a version 1.1 of Riceblogger very soon.