The best method I've found is to take your time and don't expect much to start.
This is a revised method of how I did it for those that have thrown poor Lassie into the tub, and we've all done it at least once.
Make it a session, like Salem said without water to begin. (A session of about 10 minutes)
It's kind of like training for the big day!
Cut up hot dogs are great. Take the dog into the bathroom, perhaps even leading him with a treat. Close the door. Ignore the dog completely, and leave a trail of hot dogs into the tub. Lots of hot dogs!
Than sit as far away from the tub as you can. Why? Because poor Lassie's problem is: you+tub=afraid. So by taking you out of the equation, Lassie can meet the tub on his own terms.
No eye contact. No speaking. Look at your nails, read a book, focus on something, anything than Lassie.
Lassie will probably check out the first few treats that aren't too close to the tub. Still don't say anything or make any eye contact. Lassie will keep looking at you to make sure that your not approaching to throw him into the tub.
After a few minutes of debating and making sure your not nearing, he'll probably run in grab the hot dog and back away. That's fine.
Work on this as much as you can! ANd DO NOT throw "Lassie" into the tub anymore. Take your time. Have patience.
I guarentee if you are patient, and give Lassie some time, he'll at least be able to stand the tub for the few minutes it takes to give a bath.
After Lassie, is regularly going towards to the tub to get the treats, starting throwing the treats INTO the DRY tub. Once Lassie seems less stressed, and more relaxed about jumping into the dry tub you can start using water. While Lassie is in the tub, you can approach with a handful of treats and for the first few times, just feed, feed, feed!
Let him know that the bath can be a happy, fun place!
Hope that made sense