I read many, many blogs and get ideas from them. I also keep a running list of ideas on my computer so it's easy to find and I can jot list while reading or searching the net. The weekly theme just doesn't work very well for me, but I'm so unconventional in my work and writing that I'm not surprised. Oh well!
Mostly just find things that YOU like to think and write about. There are no hard and fast rules (thank goodness) but there are grammatical rules....please take advantage of those. :-)
Good luck with your blog.