The tramps on 16s need to be ultra-tight. If you've got an old trampoline, it may not be up for the job. If there's any sign of grommets pulling out, worn stitching or other problems, get those fixed first.
1) Use a thin (3 to 4 mm), non-stretch line.
2) Pre-bend the sidebars by rigging a the mainsheet to a loop of line that runs around them:
You can easily get 1" - 2" of prebend in the sidebars this way.
3) Lace the line through loosely tighten only the first grommets on each corner to pull the wrinkles out along the sidebars and front crossbar.
4) Make a tightening pass along the rear lacings, pulling them as tight as you can (working from the corner to the center)
5) Make a tightening pass down the center.
6) Repeat in the rear, then again down the center.
7) Release the mainsheet.
This is guaranteed to get you a tramp that you can bounce a nickel off of.