When you sign up with AMCs, ask them if they will send you an email intimation before every SIP and also if its possible to change the SIP amount. In my understanding, the amount is fixed and you cannot change it on the go. If you have extra cash, just dump it in a liquid fund or a MIP fund. Read about HDFC's Flex STP also.
An alternative: I know a round about way to go about it. For eg., lets suppose you can put down 7500Rs per month into a fund for sure and sometimes 10000Rs per month. In that case, set up a 4 individual SIPs of 2500Rs, 1 week apart from each other, say on every 5th, 12th, 19th and 26th of each month (BTW, you can get a slightly better cost averaging). Whenever you have the entire 10000Rs in your account, let all 4 SIPs trigger that month. But if you have only 7500Rs that month, either add extra 2500Rs before the 4th SIP could trigger or cancel it altogether. One drawback is that you have to set up the 4th SIP once it stands canceled. Don't do it for more than 1 fund because you might forget it and it might trigger without sufficient funds in your account.
An alternative: I know a round about way to go about it. For eg., lets suppose you can put down 7500Rs per month into a fund for sure and sometimes 10000Rs per month. In that case, set up a 4 individual SIPs of 2500Rs, 1 week apart from each other, say on every 5th, 12th, 19th and 26th of each month (BTW, you can get a slightly better cost averaging). Whenever you have the entire 10000Rs in your account, let all 4 SIPs trigger that month. But if you have only 7500Rs that month, either add extra 2500Rs before the 4th SIP could trigger or cancel it altogether. One drawback is that you have to set up the 4th SIP once it stands canceled. Don't do it for more than 1 fund because you might forget it and it might trigger without sufficient funds in your account.
I understand what you means. My problem is "variable surplus amount", what I mean by this is that one month I can send Rs 10,000 and the next month in could be 2 lacs. The variability is too high and too often and I really want to take the advantage of SIP.
So far what I have thought is, I will dump my money in NRE account and make sure that I can run at least 3 to 4 SIPs without any problem.... is there any better way to do this?