On top of running endlessly through the level and continuously training to get that record-breaking run, one feature in Subway Surfers that you need to take advantage of is the missions. You will start off with earning your score normally and at its slowest pace but accomplishing mission objectives can exponentially boost the scores on your subsequent runs with a multiplier.
In some occasions, you can complete mission objectives without even being aware of them. To ensure that you secure the rewards from it as soon as possible, take time to look into the current set of missions via the missions icon on the lower left side of the main screen.
Each mission objective relates to the usual activities you engage in on each run. There are unlimited number of mission sets and the starting ones are a lot easier to accomplish. Completing a set earns you a mystery box, which can contain a variety of resources, as well as unlock the next set of mission objectives. More importantly, each set completed will earn you a star that boosts your score multiplier and the max score multiplier is 30 stars.
This is basically a requisite if you are bent on topping the high score charts. Earning a maxed out multiplier, in combination with the score booster and headstart buff that you can use at the start of your run as well as the x2 multiplier you can grab within the run, all contribute to racking up a high score easily. This should serve as your primary goal as you prepare yourself on a record-breaking run to put your name on the leaderboards.