When you get a tip from an informant about the price of any drug going up, stock up on that one drug. Buy it from different locations. You will need a car. Muscle cars allow you to get to about three places in a day. For example, if you hear that weed is going up in the Bronx, buy as much of it as available from all locations (or until you are out of money) and sell it the next day. The more product you can get, the more profit you will make.
When making a character, set his escape ability to 100. You will always get away from people if you select "Run". This is much better than fighting because that choice becomes costly because of injuries, thugs, weapons, ammo, etc.
-Try to follow the price of one drug. Buy it at the lowest cost and sell it at the highest. It is better than trying to balance out every drug. For example, the lowest cost of cocaine (when you have no hook ups from informants) is around $18,000. The highest cost is $29,000. When you see it at $18,000 buy as much as possible and hang on to it until it reaches $28,000 to $29,000. Also attempt this with other drugs as well.
-When you start a new game, make enough deals to buy the laptop. Because prices do not vary much in each part of the city's, make a note of the highs and lows of each drug. After that, start a new game. You can now avoid overpaying and find the best deal in each day.