I realize this is late, but +1 to the above post.
When I put my new headlight on (single 7" with H4 bulb), I just pulled the original low beam wire (green/black I think) to the H4 low beam. Then I realized that since the low beam was on all the time in the stock setup, I was running both the high and low beam filaments whenever my high beam was switched on. This, coupled with the fact that I ride with the high beams on all the time during the day, was most likely what was causing me to go through bulbs very quickly.
I fixed this by doing BD43's dual headlight mod, except without the nice connectors and all. I replaced the original low beam wire going to the H4 bulb with a new connection from the headlight switch. If you look at BD43's thread, you'll see what I'm talking about. With this method, whenever the high beam is on, the low beam is off.