To get the Mega Bonus, whereby you need to fill the bonus meter four times (I.e. Four lots of 100 coins), you need to collect 100 coins at a time without tripping. Repeat this four times in one run to get the Mega Bonus.
When trying to get these two trophies, which reward you for ignoring coins over set distances, it is a good tactic to pause the game at the start of the turn so that you can see where the coins are. Then, unpause it and try to get the trophy.
If you tilt to one side, it should give you more time to avoid the coins too.
You are given the option of buying multiple Explorers in Temple Run. Note, however, that there is no difference between any of them - you are much better off spending your money on powerups and utilities.