Shouldn't really need clarity and heal on varus.
Should also check into getting an infinty edge as one of your first damage items.
Aye. As you play more, you'll come to use clarity less and less (if at all), as you get better with managing your mana use. I always try to get infinity edge as my first or second item on any ranged AD. You'll notice a pretty big difference :D
I tend to spam abilities >.<
Especially on Ashe and Ezreal.
I harass pretty hard on Ez with his abilities, but I don't take clarity.
I know what I'm about to say won't be popular in here, but WHATEVER! I start with a sapphire crystal and rush a sheen. I play hybrid Ez though.
The important thing is to not use your abilities to last hit. Use them to harass and kill. You'll get a feel for it, really.
As for Ashe, only use frost shot on champs of course, and don't volley minions. Again, don't use abilities to last hit and you will see a huge improvement on mana management. This goes for APs too! They might not hit hard, but you can still last hit with your attacks. Just takes some practice and you'll be ditching clarity in no time :)