Having slept on the same futon that my parents got me when I was in 9th grade or so (I'm 28 now), I decided it was time for an adult bed. Having moved that futon several times, however, and knowing that I would most likely be moving this bed a few times before it found it's permanent home; I wanted to build a bed that could be broken down so it could be moved more easily.
This is What I ended up with.
Step 1: Overview
I had decided on a queen size so based on a mattress size of 60" x 80" plus a little wiggle room, and the platform for the mattress became 62" x 82", and sits about 12.5" off the ground. I came to this height by sitting on the other beds in the house and finding out that a comfortable sitting height was 24" and reading that the average mattress height is about 12", but I ended up buying a mattress that is a little below average, so if I had it to do over, I would have added another two inches or so.
You can see the basic idea; build two halves that are 31" x 85", the extra three inches are to make room for the braces at the head and foot to reduce sag after the two halves are bolted together. The slats sit loose on the frame, and are connected to each other with a nylon strap stapled to the bottom so they can be rolled up and moved as one.
Fully dismantled there are five components, but I haven't decided on a headboard yet, so that may be added later.
The frame is all pine to reduce cost because you don't see much of it anyway, and facade is oak.