Participating in a wedding is both a large financial and time commitment. Between travel, gifts, parties, and of course, the dress, it isn't unusual for a bridesmaid to spend hundreds, if not, thousands of dollars. So, make sure that when you ask (and please remember to ask, and not to just assume automatic acceptance), you give any details that you can right away.
First, make sure to give plenty of notice, so that arrangements can be made. If the person lives far away, it is fine to call (I know that this is the internet age, but such a conversation should be done verbally, so avoid email and texting, if possible). Second, if you are planning on having a destination wedding or something that would require unusual extra costs or time, mention it. If you are going to be asking them to pay for a round trip ticket to India, now is the time to let them know. Explain why you are asking them, but don't do it in a way that will manipulate or guilt them into accepting.
Do: Your friendship and support has always meant so much to me. I would love it if you were able to be a part of this. Don't: Well, of course I'd ask you to be my maid-of-honor; you're my very best friend in the whole wide world that I absolutely couldn't do this without! Always make sure to let them know that it's okay if they think about it for a couple of days.