From all the studies that I've seen. The amount of Saw Palmetto you take is pretty important and a lot of people don't take enough or can't really understand what's written on the bottles. Saw Palmetto ususally comes in two forms, crushed up berries and extract. Usually when you buy the extract it's pretty straightforward and will say the exact mg's on the bottle very clearly, but the berries form is a bit difficult to understand unless you know what you're looking for. In the berries or extract it will usually state 160mg of extract from 600mg of Saw Palmetto in a 4:1 ratio or some such thing. Be sure to read the ingredients for the exact amount.
Basically, you need to be sure that you are getting 320mg a day of Saw Palmetto to be effective. Also, Beta-Sitosterol is one of the active ingredients in Saw Palmetto, and from the studies it seems to indicate it is the most useful part, so if you can get Saw Palmetto with a high content of Beta-Sitosterol, all the better.