Proper method for forcing 100mb on a gigabit interface.

I have some Dell 2650's with 2 onboard gigabit interfaces. One interface connects to a 100mb management network and is receiving a high number of collisions. I am using mii-tool to force the duplex (for some reason I was getting odd behavior from ethtool) manually on each boot, however, I was wondering what a more permanent approach would be? (or if it is just a matter of embedding my mii-tool command into an init script, which one would be appropriate?) Thx, CC