It all depends on how well the lane is going and how well you're farming (minions or champs). You really don't ever want to go back to base and come back to lane without getting items because you risk putting yourself at an item disadvantage (obviously). I usually start with a ring (boots start is okay too in certain match ups, but usually the ring is better) and if I end up having to go back to base with say.. 1400g, then I take a look at the match up and how the lane is going. Am I getting out traded against a magic user? Then a Chalice is probably a good buy at that point. Am I being out traded by an Attack Damage user? Then I'd probably get a Seeker's Armguard. Am I doing enough damage and they seem to slip away slightly? Then I'd get sorc shoes. If things were going even, I'd probably get the fiendish codex and another doran's ring. YOU NEVER WANT TO GO BACK TO LANE WITHOUT POTIONS. One small misstep (even if you're winning) and you could lose control over the lane while you sit at low health just trying not to die while staying in XP range.
Hope that helps some; unfortunately, League is a game of circumstance, and while it's nice to fall into a rhythm of buying the same items each time, sometimes its just not possible to.
One of the most lolzy mistakes I was making when I began wtih Ahri was thinking that I wouldn't need pots because she has spell vamp XD
Back then I'd just Q minions as they came, so I'd get her passive ready easily but I'd burn through my mp quickly, which meant I was chugging mp pots. Since then I've learned to mostly use Ahri's Q to poke enemy champs away from minions (if I manage to kill a few of his minions with the return, even better
) while only last hitting.
Last game I outplayed a Gragas simply by playing more conservatively than him early on. He couldn't approach to do much last hitting (mostly it only happened when my minions got closer to his turret) because I'd chase him away with Ahri's Q (god I love the True Damage), so he had to rely on his own Q to kill any of my minions.
Meanwhile I could freely kill his own minions since I didn't have to get close to him to do it. He actually kept up reasonably well in kills but he'd run out of mp too quickly to keep using his Q (he had to keep spamming his self-buff to heal off the damage my Q was doing, while my passive allowed me to do the same for free :'D). I edged a small lead, which became a big lead, which became me ulting at him, charming him, and bursting him down every time he got close.
Sadly, the top and bot lanes were sucking terribly so my team still lost. Downer >:
He at least kept me busy enough that I couldn't risk leaving mid to help either lane.