I have come across a couple of ideas that make a difference. I think that this material can be very tough, but good knots can be iffy. Surgeon's knots looked especially dubious untill I came accross a Bill Nash idea that really works. He sais that a 5 turn surgeon's knot tests @ almost 100%. Even a 3 turn knot looks like crap, but he twists the two parallel tags 5 times befor he makes the 5 overhand turns. This causes the flourocarbon to seat in a perfect barrel shape. Nothing like a tidy knot to inspire confidence.
The other idea is to use silicone fly floatant to lube the knot as you cinch it down. Wow, I used to joke about marketing a special fly fishing spit surogate("use new SPIT! Perfect for when you are too sea sick or the breaking fish are rattling your nerves."). But try it, knots really cinch down tight. This is great for flourocarbon.