Combine this strategy with the fact that the highest ROI on most properties is 3 houses, and you have a winning strategy against most non-experts.
1. Do whatever it takes to get cheap sets (other than risking somebody else getting the red or orange set). This is where blues and purples are very useful.
2. Put as many houses as you can as quickly as possible.
3. Wait for desperate people to trade with you to get the oranges or reds.
4. Spread your houses across the sets you own with 3 on each property, prioritising the reds and oranges.
NB - nobody likes me at Christmas.