Sometimes a subject for a blog presents itself without having to think about it; something happens or a thought arrives in your brain and the words almost write themselves. At other times, the search seems impossible. You can trawl through old notes, leaf through the newspapers, go for a walk and nothing hits the right note. There's nothing for it, but to tie yourself to the desk (metaphorically or not, depending on whether the latter provides the kernel for a story about hostages or a magic trick that went wrong) and keep typing until a subject shapes itself. Even forming one coherent sentence can be hard on these occasions. However, if you blog regularly–say once a week–it's important not to miss your deadline if you can avoid it. 
Writing when you are struggling for words proves to you that you can do it, and the more you do, the easier it gets. The discipline of writing to a deadline you set yourself is an excellent way of preparing yourself for deadlines that a publisher might set you. Try telling a magazine editor that you're waiting for inspiration to strike when he or she has pages waiting to be printed! Miss your self-imposed blog deadline and you might think that you're only disappointing yourself, but what about your loyal readers? If they visit your site regularly and find there are no new nuggets of wisdom too often, why should they bother?
Of course, life does sometimes prevent you blogging and everyone deserves a holiday occasionally, but if you can resist the temptation to bunk off it can only do you good. And now, the garden is whispering to me that it's a lovely day outside and it needs weeding. Having blogged, I can answer the call and enjoy it without feeling guilty.