thank you both . We are going to use salt and vinegar.
It depends what kind of dye you are using. For some dyes start a cold wash cycle in the washing machine, add two cups of white vinegar and two cups salt to the water when it's 1/2 full, then add the dyed clothes and complete the cycle. You c an do this regardless of what type dye you used, it won't cause any harm, it jsut might not help. However you're probably dying cotton so you'll want soda ash. (That site also has a bunch of really great instructions on tie dying here.)
Depends on the dye and the fabric, but most decent dyes used for tie-dye are set using a solution of soda ash, or there's soda ash in the dye powder. If it's RIT dye, you're pretty much SOL.