I use a powerhead at the surface to keep the top of the water line moving. Then I hold a small net on the other end of the tank and let the powerhead do the work. Its almost like skimming grease from cooking soup, sauces, and etc. Sometime there are dead spots (no current & depends on tank size) and you would just stir it to get it moving. The trick is you want the water to be flowing at the surface and no agitation like bubbles and hob filters. Do it first and by the next water change you will have very little to touch up.
This is all assuming you don't have other floating plants that you don't want to keep and you haven't did anything that would have some duckweed submersed (like in a moss ball or junlge of water sprite) below the waterline.
