Stay in the back with Ashe. Play her like a mage. She has a little bit more HP than most straight mages, but her attacks are either single target or can be blocked by creep waves. That makes it hard to harass. Buy a Doran's item or boots at the start of the match to help avoid early ganks, and don't be aggressive. If you're in lane with someone who can stun or slow, be very careful. One stun or slow can mean your death, especially if one of the people you're laning against is ranged. or has ignite.
Late game, stack attack damage and move speed. Your goal is not to kill people. It is to slow them with your frost arrow. Because of that, be very wary of trying to gank anyone on your own or even in duos. You are tremendously useful in team fights, but if the enemy focus fires you in, say, 3v3 or 2v2 fights, you become a liability. Play with that in mind.
All said, she's an excellent choice to complement a team full of slow but hard-hitting champs. For teams full of fast champs, though, or teams that already have a lot of slows and stuns, Ashe may not be the best choice.
Yet another reason why you shouldn't lock in until everyone has chosen their champs.