Montañita is a popular destination for foreign visitors, guayaquileños and quiteños alike and as such is a relatively easy destination to get to. Regular buses (up to four an hour) stop at the Montañita bus stop above the town, heading both north and south along the coast throughout the day. The towns of Puerto López (north), and Santa Elena (south) are both an hour and a quarter away and should cost you around $1.50 to get to by bus.
From Quito the easiest way to get to Montañita is to fly to Guayaquil and then take a bus (3.5 hours, $5), however, it is also possible to fly to Manta (which is actually closer) and then take a three hour bus down the coast.
There are no direct buses from Quito to Montañita. To make the journey by bus, there are three options: Take an overnight bus to Puerto López (11 hours, $12), and then a bus down the coast (1.25 hours, $1.50); Take a bus to Guayaquil (8 hours, $9) and change there; or take a bus to Manta (8 hours, $9) and then work your way south down the coast.