This is why I find by idea notebook so vital. I use a small pocket notebook that fits easily into a jacket pocket or purse. I also have a pen clipped to the cover so I am never caught without a writing implement. A PDA or note-taking app on a phone works as well, but I find it easier to become inspired by the act of writing and reading longhand.
I have the notebook divided into two sections by a sticky note. The first section is just a list of ideas that I jot down as they come to me. I may see a magazine cover at the grocery cover that inspires a blog title, overhear a conversation in a coffee shop or just have a great idea hit me while awaiting the kids at soccer practice. I jot these rough ideas down immediately, so I don't have the chance to forget them.
The second section is for brainstorming. When waiting at a the doctor's office or DMV, I pull out the notebook and look at my list of ideas for one that looks inspiring. Then I flip to the second section and begin brainstorming on how I can approach it on my blog. When writer's block hits, I just pull out my notebook and refer to the list or the brainstorming section for an instant idea that catapults me past my block and keeps me posting regularly.