No ... there is no simple "best method" to determine the best frame size for you. Inseam measure is a reasonably reliable method to use if you are planning to purchase a bike to ride around the block, but not if you intend to ride at all seriously. Sizing by inseam basically just ensures that you will not hurt yourself when you stand over the top tube of the bike.

Determining the correct bike size for a person depends on many factors such as: Body measurements (inseam, torso, arm length, femur length, etc.), Flexibility, Riding experience, Intended use for the bike, etc.

If you decide that inseam measurement is good enough for you, keep in mind that you should not just use the number from the pants that you buy. A typical description of how to measure your inseam reads like this:

"Stand in stocking feet with your back against a wall. With your feet about eight inches apart (pedal width), place a carpenter's square (a notebook also works) tight against your crotch. It should be tight enough to simulate sitting on a bike. The other plane of the square should be against the wall. Measure from the floor to the top of carpenter's square or notebook."

Go to several local bike shops and talk to them. They should spend some time with you to get an understanding of your needs. Then they should take some measurements. And lastly, they should recommend some bike styles and sizes that are appropriate for you.

Here is a link to what Sheldon Brown had to say about frame sizing. This page also has links to several other good sites:
http://www.sheldonbrown.com/frame-sizing.html

Another set of measurements people often use to determine if a bike will fit them is Stack and Reach. There is a series of articles on it starting with this one on Slowtwitch.