From what point to where do I start measuring? I suspect the bike I'm trying to measure is a 56cm because I usually ride a 54 and this is slightly too big, but I want to be sure.
There are a few more tricks that you need to be aware of. The most important is that a lot of modern bike frames, particularly mountain bikes, have a sloping top tube. If you use the method described by @kkeilman you're size measurement will end up being too small. The trick in this instance is to get an assistant to hold a horizontal reference such as a yardstick, metre rule or broomstick aligned with a spirit level (or by eye) at the level of the top tube/head tube junction. You can then measure the size from the centre of the bottom bracket to this reference line, keeping the measuring tape in line with the seat tube.
My understanding is that most US mountain/road bikes are measured from the center of the bottom bracket shell to the top of the seat tube, European road bikes are center of the bottom bracket shell to the center of the intersection of the top tube and seat tube, and BMX bikes are from the center of the top tube intersections between the seat and head tube.
- US Road/MTB: Center to Top (C-T)
- Euro Road: Center to Center (C-C)
- BMX: Top Tube (TT)
This page backs that understanding up, with pictures.