Early research suggested that consuming carbohydrate immediatey prior to training hurt performance by causing blood sugar crash due to and insulin spike
This is not universal
Athletes prone to sugar crashing may want to delay the consumption of their immediate pre-workout nutrients until they are already into their warm-up.
With regards to strength/power athletes, some research has suggested a benefit of pre-workout carbohdrates and a recent study found that consuming 1.0g/kg (.45g/lb) carbohydrates before, and an additonal .5g/kg (.22 g/lb) during workout, significantly reduced the decrease in muscle glycogen from training. Limiting muscle glycogen depletion can be important from a performance standpoint, especially for athletes who train twice daily.
Using a rapidly digesting carbohydrate source such as dextrose or sucrose makes the most sense here as well, although, again susceptible athletes should watch for signs of a blood glucose crash.
Prior to power/strength training, I recommend an intake of 0.3-1.5g/kg (0.13-0.22g/lb) of carbohydrate with an equalamount of protein roughly 30 minutes before training.
This should optimize blood glucose levels and provide amino acids during the training session.