One way is just using a beefy diode (very beefy), but then you risk burnt wires etc.
The other way is the brightbox method. No need to buy one of those controller boxes if they are more than $30 or so bucks, because you can make your own at that price. All you need are some wires (12-14 Gauge is ideal), a relay w/socket or and a cheapie diode or two ( 99c at Radio Shack or cheaper online), plus the 9007 male and female sockets.
Easiest way: Basically all you do is run your low and high beam wire (with a diode on the high beam wire) to pin 85 on the relay, run pin 86 to ground, pin 30 to battery, pin 87 out to low beam on bulb. Also run high beam wire to high beam on bulb and common on bulb to ground. Done.
I can post schematics if need be.
If you add a second relay for the high beam side, you get the added benefit of brighter lights, as well since you are getting rid of the voltage drop due to the horribly thin stock wiring.