"What is the best and most cost effective way of boosting your income with SFI?"
I am sure many other people have already said this, but in my opinion the best and most cost effective way is by setting up a standing order.
For just a little under $60 a month, you can set up a standing order for a share of the S-Builder Co-Op and 125 T-Credits.
This is the method that I have followed. My reasoning behind this is the S-Builder Co-Op will bring in new PSA's every month and the T-Credits are for whatever you would like to use them for at TripleClicks.(Auctions,Posting Items for Sale, Time Machine, Pick the Price, what have you.) All of this will eventually boost your income. Remember, this will not happen overnight.
Obviously this all depends on how much you have to put towards SFI. IF you have less - do less. If you have more - do more.