Rent Increase
Once you assemble a monopoly on one color group, the posted rent on the title deed doubles for all of those properties.
Houses and Hotels
You can add houses and hotels to the properties on a monopoly to increase the rent further. You must build evenly; you can't put an extra house on a property until all properties have the same amount and can't put hotels until they all have four houses.
Best Monopolies
The orange and red properties (near the Free Parking corner) are often the best monopolies to get because they are among the most frequently landed on spaces and can be developed for a fair price ($100 to $150 per house/hotel)
Park Place Over Pacific
The dark blue properties Boardwalk and Park Place are better than their green neighbors because you only need to develop two properties instead of three. Boardwalk is also frequently landed on.
Strategy
It works best to not put hotels on your properties and just keep three to four houses on them. Since the the game only has 32 houses, your opponents can't build up their properties if all the houses are on your properties.