How to get smooth seams on airplane wings?

Personally I use Tenx-7r or plastic trim glue and just hold the parts together especially if it's a monobody construction.  

Make sure you get the gap leveled and straight.Sand the area lightly with a very fine sand paper then use “Perfect Plastic Putty “ .Apply it on the area then with a cotton swab wipe in a single straight motion.Let dry and sand lightly to smooth out.The product is on the internet .

A trick that sometimes helps at the wing root is to glue the top halves to the sides of the fuselage before gluing on the bottom half.

I usually use a couple of coats of liquid cement on both halves, then clamp tight to get a little squeeze out.  Let dry completely then scrape and sand to profile.  Any "gaps" left I fill with perfect plastic putty (PPP),wiping flush with damp q-tip before it dries.

If it lets go because I let dry too much before clamping, will wick in some Tamiya thin to reglue while clamped.

Gaps between wing root and fuselage can be more problematic, but can be filled with stretched sprue, thin styrene sheet, etc. then finished with putty.

Have pretty much gone to PPP for ease of use and non-shrinkage

Silicone is a no-no, you'll find paint, glue, and anything else won't stick to it, and is a royal pain to get it all off to do repairs.