On Cleverchimp's blog (http://todd.cleverchimp.com/blog/) I saw a good idea for riders of folding bikes:

To acheive consistent seat height after folding, put the seat in your armpit - the seat height will be correct when the fingertip of your index finger touches the bottom bracket bolt.

It's pretty close for me, though I find it's just a little short. I prefer to put my fist on the seat, with the thumb up. When my thumb hits the very top of the hip-bone the seat height is about right. For me, anyway :)