We went through this recently at 32 months, which sounds a lot older than the other responses I'm seeing. The whole bath was a problem but hair washing was especially impossible. One thing that worked for us was new toys as the other posts mentioned.
The other thing that made a big difference is that we started to do a lot of "bath time play" where we would take a big bowl or bucket (dry -- just pretend water) into the living room and play bath time with all of his toys (cars or blocks in addition to dolls if he wanted). We'd talk to the toys about how it was bath time and tell them how exciting bath time was and then we'd help them get into the "tub". We'd wash their "hair" and "body" play with them in the "water". Then we'd help them get out and dried off and spend a lot of time praising them for how clean and beautiful they looked after their baths. We also had him help was our hair and bodies in the pretend bath. Doing this all in his normal play area in the living room seemed to help relieve a lot of the anxiety.
After day or two when it was going smoothly in the play area we moved the pretend baths into the bathroom and did the same thing. Then, we introduced the new toys into a real bath in the bath tub and asked him if we wanted to get in with them. Thankfully he did. Once we got him in the tub it was still two more days of play washing his toys' "hair" before we could wash his hair. It took a couple of days, but we went slowly because of how scared he had become. We had tried playing it tough at first and I think that made it more difficult in the end, but I think that's partly down to his particular personality.
Hopefully we won't have to do it again, but I'm sorry to read that some of you went through bath time fear more than once... I've got my fingers crossed we're done with bath time fears for this one!
Good luck!