Where to put skill points

At minimum level 50 and above (preferably level 100+), you should be balancing your ATTACK & DEFENSE points gained. Without getting into complicated Mafia Wars math, your ATTACK points should be 2:1 versus your LEVEL and your DEFENSE points should be 1.8:1 or 1.9:1 versus your LEVEL.
Example:

If you are level 100;
ATTACK points should be 200
DEFENSE points should be 180 - 190

Keep maintaining this ratio as you continue to grow and you will have the best of both worlds, able to inflict much damage on attacks and able to defend oneself when being attacked.