It depends how much you want to send and how regularly. If you are only sending £30 or £40 then Western Union are your easiest option. They'll charge you £4.90 (if the amount is under £50), and skim a few peso's off the official exchange rate, but the money will be ready to collect in the P.I's in about 15 minutes.
If you want to send a large amount, ie, thousands or a regular monthly amount, you'd be better looking at the specialist currency /FX brokers (not your bank or local Travelex). The FX dealers all work slightly differently, some may charge you a small fee per transaction, whilst others may charge you a one off setting up/membership fee. You usually get the best rate of exchange from one of these companies as they only take a minimal skim from the official rate and are sometimes up for a bit of negotiation.
I'm happy to recommend these people, http://www.baydonhillfx.com/ I've been using them for about 5 years on various overseas property transactions and have so far been very happy with the service.
As already stated - Don't send money to a girl you haven't met yet, there are lot's of scammers out there preying on the generosity and sympathies of rich and gullable westerners. Some of the sob stories are very convincing, but don't be taken in.
