AP mages are hard to last hit with. Bad animations, not much AD.
And yes, There's a technique for last hitting under towers. It's all about experience. As LB, its possible to last hit with chains. (I never land chains on my opponent unless they're close.) I find that practicing in a custom game is helpful. Push the minion wave, and it'll push back to your tower, and you can practice farming under tower.
you're aiming for 70-80 cs by ten minutes, 100 is max, wraiths can help you get above max.
As LB you probs won't have as much cs as Karthus or Anivia, because Leblanc wants kills and to set the other mage as far back as possible, then roam to other lanes for even more kills.
Smartcast. It helps, it takes a while to get used to but it's well worth the effort.
In terms of masteries, the butcher mastery is useful, other than that you really don't want to waste gold or runes on last hitting. For leblanc, not as important. It helps alot, but not crucial, like with Karthus.