-Be a Maniac (being a fearless could work too if someone in your mafia makes you a wheelman)
-Just upgrade your energy to do more jobs per day
-find the most rewarding job in a job tier (xp-wise) and do that job a lot
-Get a lot of friends to grow your mafia and win battles
-make good use of energy pack, i.e: use it when your out of energy to replenish all your energy plus 25%, that way you can do double the job
-send energy packs to your mafia so they send you back
-when your about to get a new level try to do the most rewarding job (xp-wise) to rollover the extra xp into the next level, thus giving you a head start
-complete collection for any job tiers, thus earning some nice bonus if you do like 5% more energy or defense
-make a whishlist of the items you need to complete a collection and make presents to your mafia with the ones that you can spare hopefully they will send something back.
Well thats what I remember right now, there are a couple of things more but i guess this are the most easy to follow, at low levels (1-35) you can gain at least 6 lvls per day, at levels 36-70 will be like 3 per day... well for me atleast and I did a lot of bad investment when I started so if your in lower levels maybe you can do better.