Agree with the tips above. Consider wet mounting (look 'er up on google to learn about it - I know it sounds dirty, but it's not, trust me - just don't click on the wrong google results)
Additionally, I highly recommend not using film holders and either placing directly on the glass, or by using the Betterscanning plates. Betterscanning plates are nice because they use anti-newtonian coating, which means you won't get those drasted Newton rings that all the kids are talking about these days (or at least the cool kids who shoot large format).
And whatever you do: never.... never ever... use the auto calibration settings for your scanner...