I've used socks, traditional clips, velcro bands, and slapwraps, and they can all fail sometimes. Typically it's a trouser with a slightly shinier surface than you're average trouser, in which case you're buggered, since it probably won't work with anything reliably. Generally I've found the traditional clips to be the most reliable, and probably the easiest to pop into your pocket when not in use as well.
You could always insert a couple of bits of sticky backed velcro into the inside of the leg, so you can velcro the slack trouser leg(s) up, on the side away from bike. It may look a little odd, and the stickyness may not survive being washed, but it would having to carry anything, and would be unnoticeable when in un-velcroed walking-about mode.