Should A Blog Post Focus On One Topic.

As a general rule the answer is yes you should focus on one topic. Staying focused on one topic at a time mean the search engines can make more sense of your post and you readers know that you are not about of go off on another topic.

Of course there are exceptions to this. Often people will do update posts or "How I am doing posts" as I like to call them. These tend to wonder across topics but are useful as they often reveal more about yourself to your readers. This helps them get to know you and encourages them to return to your blog or start to follow you on twitter so these can be worthwhile posts.