I need not delve into the technical details of percent-encoding to make my point regarding keeping your URLs clean as part of your on-site search optimization, but some basic coaching is in order. I see a couple repeat offenses that bother me and I would like to discuss them here briefly in hopes of raising some awareness of the situation.
First major offense is the use of an apostrophe in URLs
Feel free to use them in the title and throughout the content of your page but do not use them in the URL structure. Search engines view an apostrophe in a URL as %27. So Google sees http://domain.com/don’t-do-this as http://don%27t-do-this. This causes issues with keyword separation and can very well cause issues in your search rankings. Some sites also have problems dealing with encoded characters as well. For example bit.ly and other URL shortening services have had issues properly linking to sites using apostrophes and other encoded characters in the URL.
If you use the WordPress platform for your site and/or blog it will automatically fix your URL for you after you type your post title as you can see in the image below.
Second major offense is the use of spaces in URLs
I don’t see spaces used in URLs as often as I see apostrophes but it is unfortunately still all too common. Spaces cause a similar issue by displaying as %20 character. Hyphens are the preferred character for separating two words in URLs – do not use spaces or underscores. Don’t argue with me, there are many reasons for this, including the actual spec that explains it.
EDIT: 5 Aug 2011 – Matt Cutts re-explains the dashes-vs-underscores thing in this Google Webmaster Help Video.
Same goes for Filenames
While I’m at it, your filenames need to follow these same guidelines. You optimize your image and video filenames to include keywords (right? you should be!) and should be using hyphens to separate those keywords as you do in your URLs.
If you currently have these problems on your site – I don’t recommend editing your old URLs unless you’re prepared to create 301 redirects for all of them. Do keep an eye on your new posts and be cognizant of the situation in the future and keep your URLs clean as part of your SEO strategy.