Decals: Some hate them, some love them, but they can really make or break a nice looking model. For what its worth, here's how I do decals.
Step 1: Gloss Varnish the area where the decal goes
Step 2: After the gloss varnish dries, break out the Solvaset. You can use other decal "settling' agents, but Solvaset is my favorite. I've been using it for nearly 20 years. These agents help snug the decals down over irregular surfaces.... like shoulder plates, for example.
Step 3: As the target decal sits in water, I put a little solvaset on the target area. Then we apply the decal.
Step 4: With the area wet, position the decal, then carefully dab with a paper towel.
** On textured/ non flat surfaces, once I take the bulk of the water/solvaset away, I sometimes score or 'poke' the decals to take out air bubbles and to get the decal to lay down. Light tamping with the paper towel and applying solvaset to "problem" areas. Gentle tamping with a paper towel or brushing with a small dry flat brush will help get rid of the wrinkles.
Step 5: After that dries, I apply a layer of watered down gloss varnish. After that all sets good there will be a matte varnish.
After all that dries, prior to some advanced weathering/paint chipping etc., I sometimes run a ultra fine sanding stick/sand paper over the area. this will knock down any wrinkles and such. Eventually, there would likely be a final matte spray varnish too. But, the final product is usually a seamless decal, free of flash (unlike many you see in GW books....all that fine painting and flash on the decals, for shame 'Eavy Metal team, for shame.) As with all things, it takes practice to get it right and mistakes will be made. Also, there might be some variation depending on the decals manufacture. I've used this technique on decals from Russia, the Czech republic,France, England,Germany,Japan, Canada and the US (and possibly elsewhere).... not to mention somewhat brittle 10-15+ year old decals (some of which will likely be on my Imperial Guard stuff). Lousy looking decals can make an otherwise beautiful model look crappy, so its good stuff for the somewhat serious modeller to know. Obviously, I'm always open to any questions, so hit me up.