Here's the way I've finally had fairly good success with masking bands: First, cut very thin strips of Tamiya tape for the edge of the stripe and apply to the airplane. Then mask around the thin stripes with more tape and masking paper.
French Canberra APC decals
French Canberra APC decals
French Canberra APC decals
Since I started using this method I've had almost 100% success (not quite, but it's better than it used to be.....
) because the thin strips conform to the curves better.