Got my nice heavy-duty window box brackets from Gardeners.com for my 2x4 railings yesterday. Definitely worth paying the extra money for them (they even have little hooks that fit over the lip of the window boxes to keep them from sliding out sideways). Here are some pictures:
I also picked up some redwood window boxes to use instead of the big green plastic window boxes (which didn't fit in the new brackets because of the little "lip hook"). So now it's time to go get dirt and some green stuff (flowers?) to put in the boxes.
I realize that it's pretty late in the season to start growing something (especially with as hot as it's been lately), but I haven't had a whole lot of time to devote to this (and I didn't decide to do it until mid-July). At least I'll get a month or two's worth of enjoyment out of trying to keep the stuff alive (I have so-so luck with past plants because I forget to water them).
I think I'm going to add a pair of hanging baskets on hooks at both ends of the porch (using the posts on each outer corner). Those brackets are easier to find (and they don't have to be as sturdy) and I can buy pre-potted plants (at least in order to get started this year without too much fuss).