Don't underestimate the light blue and purple properties - they're cheaper to buy and houses/hotels are cheaper too, so you can develop them faster. It's much harder to get a monopoly of dark greens or dark blue. Don't buy Boardwalk or Park Place unless you've got a lot of excess money.
Build up houses and hotels as soon as you can, even if you have to mortgage some of your other properties to raise the cash. Don't wait until you have "excess money" to buy houses - that takes too long.
Also, remember that the orange and red properties tend to get landed on the most - because of people being released from jail. So buy and build on the light blues first if you can, then think about the orange ones.
Are you playing with "auction" rules, or does unbought property stay unsold? You have to think strategically about what properties your opponent needs. Sometimes you have to buy a property you don't need just to keep it from the other player.
Also, how many players in the game? If only 2, trading properties isn't really an issue, but with 3 or more you might need to trade with other players to each get properties you need. Trading away a dark green for a light blue might not seem to make much financial sense, but if it gives you a monopoly, you can get your hotels up faster.