Hydrostatic weighing is the best. Calipers and the electrode ones are alot cheaper, but have a wider error of margin. You might find someone who has no idea how to use the calipers correctly, and it seems that NASM, ACE, and gyms all have different ways to calculate it. NASM's sites are probably the best. The electrode one (I forget what it's called) can be skewed by hydrated you are.
I have the calipers done or just guesstimate. I think most people in here guess, and they're probably within a couple percent.