Here is a pretty simple way to get extra experience points on your Mafia Wars character.
1. Make a new account.
2. Level it up a few times to level 10 and put all your points into stamina. Once you are level 10 or 11 you will be allowed to attack random people.
3. Find out the link to your first account's mafia wars profile page. To find it, have a friend (or a second account) log into their "My Mafia" page, right click on your name on their mafia list and click "open in new window." That link location is the location to your account. Bookmark this link.
4. Now, on your second account, open up the bookmarked link and attack your own account over and over again. You will lose every fight, but as a result, your first account will get experience points for winning the fight on defense.
If you are a high level character with a full mafia and good stats, you can use this strategy to list your first account on the hitlist over and over again. If you can win the majority of the fights, you will get a lot of experience points from doing this!
Alternatively, if you have 2 accounts, make one account only have 1 mafia member - that member being your other account. When first account finds items from doing jobs, your other account will get a few exp for "finding" the item. I find that the "Collect Protection Money" job in the Associate tier to have a really high drop rate. Do this job a few hundred times and your main account will get a few hundred extra experience points as well!