I didn't carry my dSLR everywhere by any stretch, but I always have my iPhone and really like some of the shots I got with that.
I set up personal rules for myself: I'm like that. I personally clicked the shutter on every one of my Project 365 photos or, on one occasion I think, set up the camera and had my dd click--but I had set everything up (it was a photo of me and the self-timer wasn't working well for me in that instance). I took a photo every single day, no exceptions. There were days it was late in the day and I hadn't taken a photo, but it only takes a second to snap one, and I always did.
I did try for a mix of subjects, but that's probably because I'm easily bored. I know of photographers who photograph everything they eat for a year, for instance, but I don't think that would work very well for me! When picking which photo to use as my picture of the day, I often looked at my recent picks and tried to pick something (or someone) different. But I didn't always look and occasionally had two of one person in a row or something like that--oh, well!
Layouts: I went with two page spreads, one page per week, so my book--it's on order!--is 53 pages long. I mixed up the templates I used, but they match across spreads. I like variety: that worked for me. If I were to do it over, I might go for a slightly more homogeneous look--maybe more compatible templates flipped around different weeks? Maybe a common background. But maybe not; again, I like variety.
I didn't journal the first few weeks and sort of wish I had. I almost redid them, but decided against it. I do prefer the weeks I journaled, though.
I wish I'd kept track of what word art I'd used throughout the year; I often wondered: Did I already use that quote or title? But I was too lazy to go through all the previous pages to make sure I hadn't. I would keep a list next time.
I have some reflections on the project on my blog
here.