Image is low contrast
Ensure frame is cropped before applying action, especially if there are parts of the negative carrier visible or excessive film base, sprocket holes, etc.
Shadow detail loss
Ensure color base was properly sampled. In some cases it is possible that the sampled base color is not the lightest tone in the scan, use a Threshold layer to identify the lightest region (lowest density/deepest shadow)
Color cast / Highlight Clipping
If there's a visible cast that cannot be negated by any combination of the Y and M layers, try lowering the opacity of the 'Channel Align' layer. Also ensure no color channel of the .tif scan is clipped by referring to the histogram.
Image is low saturation
It can happen, especially with Portra films. To fix, first achieve a neutral color balance, then use the Saturation Fix action included in the action folder.
Not all scanners are the same, through my testing I've noticed linear scans from different scanners having different saturation characteristics.
Also, your scanner may not set the color profile when saving, if so, you have to manually set it (I recommend AdobeRGB).
My scans don't contain areas outside of the frame
If you can't sample the color of the film base, then you might be able to get away with it by selecting the absolute lightest tone in the scan (the lowest density/deepest shadows), if your image is too low contrast for that, you'll need to rescan and leave in a bit of area outside of the crop.
Not getting good results no matter what
My method may fail if there are unexpected problems, these are many, but usually identifiable. The worst case scenario problems are usually: improper scan, clipped information, (very) badly exposed negative, crossed curves due to bad film processing/storage.
Photoshop CS5 or older
Make sure the sampled color is set as the Foreground Color.
If the image looks uninverted, it is because Photoshop makes a clipping mask of the inversion layer, this can be fixed by releasing the clipping mask. There is a fix included specifically for this scenario.