The Easton Shaft Size Selection Chart is a good starting point. This chart is in most catalogues and is available at your archery pro shop. Most dealers also have the software for their computers that can provide detailed information on shaft selection as well. In general, if you are a good release shooter looking for target arrows, you can usually safely select arrow one to two sizes weaker in spine than the Easton Chart specifies. However, if you are a casual or average shooter or simply choosing hunting shafts, I prefer to go with the recommended size shaft.
Stiff shafts handle broadheads much better than weaker shafts.
Finger shooters, in general, should not go to smaller than recommended shafts. The best option is to follow the advice of your pro shop, especially if they have knowledge of your shooting skill level. Each of these factors impact significantly on the stress applied to the shaft upon release, specifically the amount of bend which occurs in the shaft as the bowstring accelerates the tail of the shaft against the resistance of the point. In turn, these factors determine how well the arrow and fletching clear the arrow rest and how soon the arrow shaft dissipates the bending and vibration caused by the rapid acceleration. The following tuning instructions are intended to help you minimize the negative impacts of the shot on your arrow flight.