You need to make sure your E Malefic Visions is on a minion when you last hit. This gives you mana back. That along wtih Doran's ring should give you decent mana. BUT no mid laner can just spam abilities (without blue buff) wihtout running out of mana quickly. Catalyst will help too. Also I have started doing a second starting item, The Dark Seal, on my first back to help. This is pretty standard if you watch pro play as well to buy a second starting item to help with mana problems.

Hope they help you get wins!