I'm doing it just with saving money from my take-home pay at work.
I make about $32,000/year, and I live very minimally. One-bedroom apartment, (I could have gone $150/month cheaper if I'd decided to split a 2-bedroom with a roommate, but I decided against it,) low-cost car, I live very close to work so that I don't have high gas costs, I cook everything from scratch at home rather than going out to eat or paying for food at work, I skip going out for drinks (alcohol is expensive,) I don't have cable (just internet and a computer, where I get all of my entertainment from,) and I keep my budgets for clothes, entertainment, and basically everything at the bare minimum that I possibly can. All of those weekly $10-$20 differences add up over the course of the year.
I've put away $8,000 in the last 12 months, which is about 2/3 of the total cost of SRS in Thailand.
Work hard, and make sacrifices. That's about the only advice I can give.