The number one tip is don't drink coffee before decaling.

Gloss coat before decaling (if painted) I use Micro set when putting on the decals, it gives you time to adjust them especially if your doing individual letters and numbers. A foam cradle to keep the car straight helps getting the decals straight. Reference the lines on the car, ribs for example help to get the decals straight.
Optivisors help in seeing, to get them straight. A Qtip to soak up excess fluid.
Walthers solvaset to snug them down.