Having your gear setup up properly is very important for the ultimate control in all conditions. A poorly setup board and rig will perform badly as well as being hard to control. Some signs that you are poorly setup are spinout, tailwalking, difficulty staying upwind, difficulty planing, and if one or both arms are getting over tired from pushing and pulling on the boom While many of these signs can be attributed to other problems as far as technique and equipment but your setup can contribute greatly. Unfortunately sometimes it requires good technique to determine what the proper setup is for your board and rig. Lets assume for this article that your technique is good.
There are basically five areas that you can tune to get your setup set properly; fin, mast track, harness lines, boom height and strap position.
First you need to determine where you want to place your straps. For early planing and heavy weights it is good to place them into the forward position. For speed and tighter control in turns, place them into the rear position.
Once you have the straps into position you can place your fin into the fin box. The position of the fin is determined by the placement of the rear strap. It should in line with the rear screw of the rear strap. If you do not have any adjustability in your fin box then don't sweat it and pop your fin in. The leading edge of the fin should never precede the rear strap screw, this can and will induce spinout.
Now it is time to set the boom height on your mast. A good starting point is to set it at the same height as your collar bone. This will cover about 70% of the conditions. For lighter wind, you can set them slightly higher so that you can put more weight on your booms for earlier planing, for heavier wind you can set them slightly lower. The boom height effectively determines how much weight is placed on the board, and should never be placed lower than the sternum and higher than your chin.
Your harness lines are probably the most important factor in getting everything in balance. While on the beach locate the center of effort of your sail by finding the balance point on your boom. Set your harness lines equidistant from this center point. This is a good start but will need some tweaking once under power. Sail with them in this position and feel the pull on each arm. The pull should be equal on each arm. If the pull is greater on your front arm then move the lines toward the front, if the pull is greater on your back arm, move the lines toward the back. The adjustment should not be more than an inch in each direction. In addition the harness line separation should be no more than shoulder width. The closer they are the more sensitive the rig will feel and you will be able to sheet it properly.
Harness line length has been a big issue of debate and all I am going to say on that subject is that they have to be long enough to not depower the rig to windward while under sail and short enough so that your arms do not get tired and your butt shouldn't slap the water. The adjustment I use is about from the tip of my pinky to the tip of my thumb. Thats about right for me. Everyone else may be different.
Finally, once all else is setup you need to set your mast track. Your mast track should be set such that the center of effort of your sail falls at the midpoint of your straps. If this is set properly you should have equal pressure on both legs while in the straps. If your front leg has more pressure (called front loading) you need to move the mast track back, if your back leg is loading more, move the track forward.
Well now your are set and ready to sail. This may seem like a lot to do before sailing but do it once and you are set for the life of your equipment. If you get a new board or new sail you will need to do it all over again. Write down your adjustments for the different conditions and keep a log. It is the only way to truly find your balance point.