Control Tweaking

In addition to 2 (or more) boat training, open meetings and coaching, which are well worth getting involved in, try this, it's very visual.  

Set your boat up on land in a suitable breeze. Not to windy or the boat may blow over. Attach a piece of shock cord to the end of the boom and to the transom somewhere. The shock cord should be partially stretched when the sail is 'set' on a the point of sailing that you are interested in. 

Now try pulling (and easing) things. Outhaul, cunningham, vang and even the jibsheet. You will notice that the boom will move both in and out. If the boom moves closer to the centreline of the boat, the 'power' of the rig will have been reduced. If the boom moves out the power of the rig will be increased, causing the shock cord to stretch more. There are occasions when you will want to add power and decrease power on all points of sailing. This technique is very visual and will let you see first hand what is happening. One of the most surprising things to observe is the effect the jib has over the power of the whole rig. Not just sheet tension, but halyard too. 

Try it. It even works when the wind isn't constant. You will see the effect immediately.

EDIT: Use the advice given above and in various books as the basis for these trials. They are all correct, but all classes, rigs and sailplans will react with with varying results to each control.