I think the key to effective improvised prusiking is to have whatever is hitched to your harness able to move up instantly and grab. So on the harness I'd use a gri-gri (but this won't work for prusiking up rappel ropes), one of the the %$#@&ing guide plates I just hated on above, or, my favorite, the misnamed Garda hitch, better-named the alpine clutch, fabricated from a pair of carabiners.

Hanging from your harness, you can comfortably futz with whatever knot you are using for the foot loop and move it up. As you step up, you haul rope through whatever you are using as a clutch and you are instantly stable and hands-free at your next position.

To use this system with the Garda, you have to have two nice-sized lockers, which almost everyone has on them nowadays, a prusik knot cord, which most climbers also have for a rappel backup, and enough slingage (say three over-the-shoulder runners) to arrange a foot loop. This last item can be the rub; it isn't unusual to see a second starting up a pitch without anything that could be used as a foot loop. Of course, if the party is using cordelettes, there is no problem.

In almost everyone's hands, I find guide plates provide a very inferior belay for seconds who want to climb the pitch without being pulled, not to mention the fact that the plates encourage belayer inattention. (Eat your lunch while belaying! Change clothes! Zone out until your second takes a legitimate leader fall from all the slack you've left!) Unlocking a guide plate when the second is hanging free can be very difficult. In some cases, if the plate is pulled against the rock in certain ways, its ability to tilt may be blocked off and unlocking will not be possible, and then a full-on belay escape is the only option for lowering.

The devices are a solution to a problem that didn't exist. But more and more climbers are learning outdoor climbing while belayed by these plates, and so don't even know what a good belay is like. They expect to end up on tension if they step down, and accept the idea that if they have to reverse lateral or diagonal moves, they are likely to be pulled off.

Wonderwoman, if you are going to prusik up double ropes after a belayed fall, it makes more sense to prusik on just one strand and be belayed with the other strand. That way you don't have to tie back-up knots in the rope to protect yourself in case the prusiking system fails in some way.