One good way of getting rid of duckweed is to buy some Austrailian Rainbows. They think its candy. I'm actually growing some in a spare tank and throw a large net full over in my 75 gal (6 rainbows) they eat every peice in about two days.
Manually moving them is the best way. Plus, less lighting helps. I discovered my tank choked full of them when I got back from a one-week trip. My family forgot to turn off the light after it has been on for 10 hours, and so they more than left it on for 15. When I saw the tank, I couldn't even see the bottom. Plus, they sent out these long strands of roots that made the entire thing even thicker! It was, however, much easier to remove with all these roots cus then they were all bundled in one package
I do like a few of them, but, my tanks are getting so that between the riccia, pennywort and the duckweed I can't feed my fish :shock: I guess I'll have to net the majority of it and do a removal weekly to keep the amount down to manageable amounts.