Using Decal Stripes
Be sure they are of good quality (Microscale, etc). If they are cheap, and are of a light color like yellow or white, and you put them over a dark color like black or blue, that dark color will show through.
To get a linear decal straight (any decal, could be a road name, a stripe, etc),
be sure to have a good coat of clear gloss over your color coat
use a LOT of water on the model as well as the decal
Lay out the decal in the general location it needs to go. Then gently pull it from side to side until it is straight. Keep adding water as you go, making sure the space between the decal and gloss coat is wet with water.
Once you have it where you need it, take the edge of a tissue and lightly wick up the excess water.
Once you have that excess removes, blot it some with a lightly dampened (not wet) paper towel.
If it looks good, apply decal setting solution.
Once this dries (wait a day or so), re-coat the whole thing with clear gloss, then weather, and clear flat.
--Jon