Forecast Notes 

First let me state the obvious.  Waves are created and affected by the weather.  Therefore predicting waves can be no more accurate than the weather forecasts themselves.  And we all know how unpredictable the weather can be.  That said, let's see how we use those weather forecasts to predict waves.

There are three main "tools" I use to predict wave conditions.
National Weather Service (N.W.S. [No relation to N.W.A.]) Marine advisories.
Surface Maps.
N.W.S. Wind/Swell Period forecast.

N.W.S Marine advisories.
This is a no-brainer.  Spelled out in plain english, are the latest conditions, and forecasts for wind speed/direction, height of seas, and occasionally, approaching swells.

Surface Maps.
Low and High pressure systems, which create the winds, which create the waves, which create the smiles on thousands of surfers faces around the globe.  Available from several sources, surface maps are often the best long range forecast for wind conditions you can find.  Isobars are areas of equal pressure surrounding the high and low pressure systems, and are represented by those nifty little lines around the "H"s and "L"s.  Wind flows along these lines.  Clockwise around a high and counter-clockwise around a low.  The closer the isobars(lines) are packed together, the faster the wind will be flowing along those lines.

N.W.S. Wind/Swell Period forecast.
In my opinion, these plots alone, provide enough information to make a relatively accurate surf forecast.
Based on algorithms generated from years of research, the N.W.S. provides possibly the most useful tool for predicting surf conditions.  These charts are faxed in, in tif format. There is a huge repository of these tif files at http://tgsv5.nws.noaa.gov/pub/fax/. I have taken a few of these files and cropped/rotated/appended/scaled them in one handy dandy web page for your and my convenience.  They may look a little confusing at first glance, but fear not, they are much simpler than you may think.
First, we have the wind speed/direction-sea size plot.

In brief, the "shaft" points in the direction the wind is blowing, and the "barbs" and "flags" represent the wind speed.  Each "full" barb represents 10 knots while a "half" barb represents 5 knots.  Each "flag" represents 50 knots(these are rare). Also appended to each flag is the predicted sea height(in feet).  That's right, how big the seas are predicted to be on that specific date.
Next, we have the swell period/direction plot.

The arrow points in the direction the swell is heading, and the number represents the "swell period" (i.e. frequency) in seconds.  The larger the number, the longer the period, the more powerful the swell.

Several people have made attempts to come up with formulas using data from these plots. Some of the best are at Walts Surfing Zone.

SUMMARY

There are several other tools available for predicting waves on the net, many of them available from The Forecast Page, The above information should be enough to make the best possible prediction of the surf, but in my opinion, it is best to become familiar with as many resources as possible.  We (the online community) are all too familiar with access problems, and data unavailability, and these other resources may be all you have from time to time.

I make no attempts to create a cookie-cutter type formula, as every beach is different, and nearly every beach gets good under different conditions.  I only attempt to forecast the waves for the areas I am familiar with, and provide the resources for predicting the waves in other areas.

Happy hunting!