Ok, here's a two useful things that can be used ANY time
1. Portable Shop
Touch the map, then touch services then touch Weapons, there you can buy more Health, Pistol, SMG and Shotguns
(Handy when surrounded by Police and running low on ammo.
You can also Buy cars to but they're mostly at parking spots.
2. Buy Less Cops
Touch your Map then Services then touch Corruption. Here you can "buy" less badges therefore Police won't come after you. It costs $1,500 to get rid of one badge (kinda a rip off but still makes it easier when your on a mission)