Ok, I really don't understand why everyone is saying do the last missions for extra points. It is all about the multipliers, i.e. blow the fuck out of as many vehicles as possible. I completed the game on hard and finished on about 775,000 points. I found re-doing the car missions was netting me about 30k for each one. This involved getting the car back to the garage as quick as possible and then going round blowing up fuel tanks. That was good but then I thought about the first few missions and how much time you had left to get to the save zone once you had done the initial objective.
Over 800K in one mission
This is what I did. Go to the "Lost in Transit" mission which I believe is the first mission that you start on after your out of jail. If you cannot remember where that it is, it is in the bottom right hand corner of the map in Oyster Bay near the docks (see map below). Start it up and do the initial objective as quick as possible, trying to get head shots if you can. Now drive back to near the save zone but drive past until your in an area with plenty of cars which should be no more then a block or two away. Now make your way down the street with the shotgun blowing up fuel tanks, but making sure you never directly aim at a person or especially a police officer or police car in case you shoot them by accident. This will ensure your wanted level never goes above 2 stars which means the police will not shoot at you and just continue to follow you down the street. You need to ensure you string these explosions together so knowing where the fuels tanks are on certain vehicles is a must. Also if your half way up a street and there are no more cars left either turn back on yourself or leave the last car until you are well past it which will mean your multiplier bar will last that bit longer (see diagram below). I got up to 85+ multiplier before I saved
and I still had quarter of the clock left. My final score was 882,150 for a single mission
and put me into second highest score for that mission. Check my score of the Leaderboard if you like.
Make sure you are fully loaded on as many weapons as possible. Go to gun shop and/or Harry's Army and Navy first if you don't have much ammo.
Where to do it....
Little diagram below should help.