How can one make Fluoxetine (Prozac) more water soluble?

Strictly speaking you can not change the solubility of a substance in pure water without altering its chemical structure, but then it isn't prozac any more. What I think you are getting at is how to get it more soluble in an aqueous solution (as apposed to pure H2O), in this case you can add an acid to the water, the free H+ ions will bind to the nitrogen in the structure so the nitrogen will then have 4 bonds and will be positively charged, this positive charge causes the molecule as a whole to become more polar and hence more water soluble. It is pretty much the same as what you asked but it is important to distinguish pure water and water+acid.

My question is why would you want to make it more water soluble? In general for medications designed to act in the central nervous system it is beneficial for them to be more lipophilic and hence more hydrophobic (less water soluble), this enables them to cross the blood brain barrier more effectively. But this can be taken to far because if you make a drug to lipophilic, and hence hydrophobic it will not go into solution in the GI tract and hence can not be absorbed into the body. Its actually a pretty delicate balance medicinal chemists have to deal with.