Dont bother with a last whisper if the enemy team isn't bulky, go for Razor or another Phantom Dancer. If your team isnt a total failure Ashe should never need any magic resist or armor items they take too long to build and it cripples your damage output severely.

When i play Ashe i just like to creep in the back and step forward for volley, try to throw your arrow from the side of the fight ex( fight in mid, go down to bushes in river befor shooting arrow up, or you will just peg their tank and it will be wasted) remember to keep your frost shot up at all times during teamfights and volley every CD.

I level hawkshot more than frost shot because at the start, assuming your solo-mid you wont need to slow anybody, if your in lane go ahead and switch between hawkshot and frostshot.