How to improve visualization?

It's just a matter of practice. The more you play blindfolded games, the better your visualization will become. It takes time to get used to it.

Probably your right. I've been playing a couple blindfolded games a day, and have seen slight improvement

It's like that. A matter of practice, experience, and state of mind. 30 years ago I could play simul blindfolded (I never tried more than six games simultaneously, although it did not seem hard to me), but right now I'm sure that i will forget quite a lot if I attempt the same thing.

If you don't mind me asking, how important do you think visualization is in OTB chess? Part of the reason I started practicing blindfolded, is having a disappointing tournament performance, and feeling like many of the mistakes could have been prevented if I had a clearer picture of the board while calculating.

Also It feels like every master is also good at blindfolded, so It feels like it has to be important : D

 I think it's useful, but not terribly important- and masters do well in blindfold because they have a lot of exprience in their pet setups.