The Bioelectrial Impedance method is good, but pay attention to your client's health. The BEI method is contraindicated for those with some surgically implanted appliances (i.e. pacemaker).
I use the Durnin 4 fold caliper method (Bi, Tri, Sub Scap, Illiac Crest) Method. The main idea for a trainer is to use the method to make sure a client is loosing fat or gaining muscle. Therefore, the measurements are realitive to one another and the absolute scale isn't that important for most clients. For example, client measurement 1 is 15% then 13% then 10%. So you know you're going in the right direction.
Hydrostatic weighing is still the gold standard.