Getting the $1T and $10T achievements in Mafia Wars

Pretty much just play a long time, but if you invite a lot of friends and buy a lot of Mafia Mike's it's a lot easier.

Here's one trick that should help.

The Boss Fights in New York payout rewards based on a % of cash you have on hand.

Street Thug - Giancarlo Morillo 0.30%
Associate - Confront Detective West 0.36%
Soldier - Confront Luis Castana 0.39%
Enforcer - Confront Agostino Cleto 0.54%

Thus, the more money you have, the higher they payout. So you can ladder your way up to pretty high payouts with very little risk if you have your character developed enough that you always win these fights. It will cost you a lot of energy though.

It also doesn't hurt to be a Mogul. Change classes if you have to.

Finally, try really hard to vault the "Death By Detergent" collection. It reduces the bank fee from 10% to 7% making it less money you need for the achievement since it only counts the amount vaulted after fees.