As stated, Sal Soda is different than baking soda. The same thing can be purchased from most grocery stores as Arm & Hammer Super Washing Soda (a laundry detergent). A little (not a lot!) dish soap seems to help cut the grease in the fat, as does changing the water at least once. The fat is largely responsible for the yellowish look that some skulls have after boiling. The fat liquifies and is reabsorbed into the bone.
Another worth while purchase is the 40% hydrogen peroxide and Basic White (both available from taxidermy supply places or hair salons). This stuff, mixed into a paste and applied after the skull dries, really brings out the nice white skull you want. Leave it on overnight and wash it off. Its about $20 but will do many skulls.
The car wash trick works well too, saves alot of scraping. This year I am going to try it before boiling also to see what I can blast off the easy way. You will get soaked doing this though! Good luck, a nice skull mount is much more interesting than just a plaque in my opinion.
