I know a lot of the condo and apartment blocks used to rent out spare parking spaces to the general public. I know the Lincoln used to for around $120/month as well as the Gotham (covered) on Montgomery and I think the Wells Fargo on Pavonia.

Worth calling them:

http://www.kannekt.com/15/96apart.htm

http://www.kannekt.com/15/97condo.htm

Also if you are working in JC, a lot of firms have agreements with the local parking lots to provide deep monthly discounts (up to 50%). Check with your employer.

Also watch Craigslist for private garage rentals.

There was some info on parking in the Newport Mall parking deck for HP residents in this thread:
http://jclist.com/modules/newbb/viewtopic.php?topic_id=11999

Not sure how up to date this info is but it says $85 / month which is a good deal in my opinion.

I am moving to JC in a few weeks and am looking for parking (preferably indoor) near Hamilton Park, Grove St or Newport Mall. I already called Central Parking, but the prices are too high. Any suggestions?