MCS

Joining cat clubs/flash cat clubs, is the fastest way to gain enough money to upgrade.

 

you buy a virtual cat and you give it to a cafe so you can all abuse it by hitting it and milking it

 

Piggy Banks from hunts are helpful, specially if you save up a little and use them all at once...or just forget you have them...then use 4K at once
You could always try and find open cat cafes, or ones that only cost a chibi to get into, if you can't afford to buy a cat from the store or from another player (which usually are 4 bentos or 4mcs depending)

 

Join a cat club and be very active.

 

Sell mcs items for bentos and sell bentos for dvp. This will work when you have much cash in hand though. You can also do what other players suggested above. But the best thing to do is joining a cc club. Good luck!