Pokémon X Y – How Many Pokemiles Earned by Each Method in Pokémon X Y

Wonder Trade

According to Bulbapedia:

Wonder Trading will give the player Poké Miles depending on how far the other player Wonder Trading with them is from their location; the farther the other player, the more Poké Miles will be earned.

I live in Italy, and if I trade with another Italian player I will get a bunch of 10 or less Poke Miles per trade. However, a trade with someone really far (Japanese players for me) can give you a hundred of Pokemiles, more or less.

Streetpass

Poke Miles are calculated by the distance between your home location and the other player's home location. So by Streetpass, you're mostly getting people from basically 0 miles away.

Streetpass is generally pretty useless for Pokemon XY for this reason, and I would personally suggest ignoring it. If you want more, you could make sure to walk/drive past Streetpass relays, but your time would be much better spent simply wondertrading a few pokemon to get the Pokemiles. Since a good percentage of wondertrades are from Japan and the US, which are of course a good distance away, wondertrading will very often yield a fair number of miles.

If you are planning a trip to a popular tourist location, if you carry your 3DS in your pocket, you will likely see many Streetpasses from other countries (this could be "Super Effective" for pokemiles too).

Source, this answer.

I think this should be true for online battles, too.

Pokebank

Poké Miles can also be earned by storing Pokémon in Pokémon Bank.

Poké Miles are awarded for the length of time that a given Pokémon is deposited into Pokémon Bank. The amount of Poké Miles earned by the player increases if more Pokémon are deposited into Pokémon Bank.

This is actually another good way to farm Poke Miles, the more Pokemon you store on the Bank, the more Poke Miles you get.
You can easily get about 1000 Poke Miles every month, just with a bunch of full boxes.