Relay kits which draw directly from the battery through heavy gauge wire are the way to go. Some kits draw from the alternator, increasing the voltage to the bulbs for increased light output. But this also allows surges and spikes to hit the bulbs as well, shortening life. Look for 10-12 gauge wiring from battery to relay kits - the larger the wire gauge, the less restriction to current flow.
I also like the Philips Vision Plus http://www.powerbulbs.com/
which tested out at the top for 55 watt bulbs. I can see the difference.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E