I have toyed with various electrolyte drinks over the years and have found something to recommend each of them. The most cost effective is Gatorade powder, and one study that I came across rated it just as effective for recovery as other more expensive, high-tech sports drinks.
I should probably do some research regarding the appropriate levels of sodium and potassium since these two vary widely across the spectrum of sport drinks available. All I know for sure is that potassium and calcium are required for proper assimilation of sodium, and of the ones I've tried only Emergen-C, Zipfizz and Nunn contain calcium; however, if your diet is well balanced it shouldn't be an issue. Here's a sample:
Drinks with calories:
Gatorade: 50 cal., 90 mg NACL, 25mg K.
Power Bar Recovery: 180 cal., 500mg NACL, 24mg K.
Emergency-C: 25 cal., 60 mg NACL, 200 mg K.
Zipfizz: 10 cal., 60 mg NACL, 905 mg K.
Nuun: 3 cal., 180 mg NACL, 50 mg K.
Drinks with 0 calories:
Fizzies: 150 mg NACL, 90 mg K. (Root beer is my favorite)
Lite salt (1/4 tsp): 290 mg NACL, 340 mg K.
A serving size for Gatorade, Emergency-C, and Fizzies is 8 oz., whereas Zipfizz, Power Bar and Nunn is 16 oz.
As you can see the ratios are all over the map, and I haven't been able to acertain what is optimal, though maybe that isn't an issue because they're all helpful towards recovery.