Dave,
Very nice work! I like the way you painted your tank, the first time I saw it I thought that it could be too irregular pattern, but I see that it works!
The only thing I think you could change is the decal, it looks to me like you haven't use matt finish yet:
Here is a way to solve this problem: First paint the model as you normally would. Now, before you would attach the decals, overpaint the model with gloss varnish. Gloss varnish is a transparent paint that will make your model glossy and shiny. You can buy varnish where you buy your regular model paints, but nothing beats regular floor polish (Johnson Floor Polish)! I am not kidding!
Now, let the varnish dry, and attach the decals. They should adhere much better to the glossy surface. Let the decals dry for at least 24 hours.
Of course you don't want your finished model to look all glossy and shiny. Just spray over the entire model with flat or semi-flat varnish. This way you restore the flat sheen of the model and at the same time "seal" the paint and the decals.
Sometimes you wish to remove a decal. As long as the model is not coated with varnish, you can try to rip off the decal by using a piece of masking tape. If this doesn't work, you can use household vinegar to moist the decal. After a few minutes it should be fairly easy to get off. Be warned though - the paint may be damaged by this, so take it easy!