I wanted to enhance the lighting of my Prodrive UK300 lights. According to Daniel Stern, a 10% drop in voltage to your headlight bulb causes a 33% drop in light output (http://lighting.mbz.org/tech/relays/relays.html). I measured the voltage drop at the headlight connector and found:
1) with the low-beams on, the low-beam ground pin had a 1 volt drop compared to battery ground.
2) with low-beams on and high beams on, the low-beam ground had a 2 volt drop compared to battery ground and the high-beam ground had a 1 volt drop compared to battery ground.
3) the common 12V pin had a voltage drop of 0.2 volts compared to battery.

The measurement matches my visual impression: when you turn on the high-beams, the low-beams dim quite visibly.

I constructed a relay harness. Attached is a schematic. With this set-up, one relay switches power to the common power pin and the other relays switche the low-beam ground and the high-beam ground pins. This is my preferred lay-out, but is a bit over-kill.

After installing the relay harness, the maximum voltage drop that I measured was 0.1V.

Parts list:
(2) or (3) 30A Bosch relays (make sure you get automotive grade relays)
(2) or (3) relay sockets
fuse holder with 30A fuse
12 gauge wire of choice
wire loom of choice
shrink tubing
(I purchased all from www.partsexpress.com)

If you need the sockets and plugs, you can get them at http://suvlights.tripod.com/suvlightscom/ in the "Relay Harness" section.

Shown are three possible schematics. The first one shown below requires 3 relays but gives you the protection of a voltage line that is normally off along with normal headlight operation.

You can also use an easier 2-relay setup. It's just like the one above, except that you delete the power relay and tie the battery straight to the B+ line at the headlights. This one keeps a voltage line open to the lights all the time, which you may or may not like from a safety standpoint. But all lights operate as you would expect.

Version 3 has a relay to turn the power off at the headlights. However, with this version, if you flash-to-pass, both the low-beam and the high-beams will flash. (Normally, only the high-beams flash.)

mulder over at www.nasioc.com pointed out an even easier way to upgrade your lights without relays. Since most of the problem is in the poor low-beam ground, you can take the low-beam ground wire from the headlights and tie it straight to a nearby good ground.

For another boost in power, take some good gauge wiring and tie the B+ terminal at the headlights straight to battery with an in-line fuse.

This is a nice cheap set-up that will also have the same flash-to-pass problem as version 3 above.