Actually, according to the official rules, PaSTE's strategy doesn't sting as much as he might suggest, but it does hurt quite a lot. It's not effective in a one-on-one game, but if there are multiple players still alive, you can at the very least, cause your creditor some grief.
What causes the major hurt is that your creditor has three options for each property when you declare bankrupcy to him/her:
A) Pay the mortgage right now
B) Pay a 10% "transfer fee" then later, unmortgage for mortgage value + 10% again
C) Pay nothing now, then unmortgage for mortgage value + 2 seperate 10% mortgage value fees.
So either you deny them use of the property for a longer time or they blow a lot of cash at once. It's not a Super Weapon, but if your creditor is barely in the game, you keep him/her from becoming much stronger from your property.
In conclusion, Monopoly is good, I need to get more sleep, and boredom is a Good Thing™