You may already do this, but taking the enemy's wraith camp can be a nice boost your your CS, ideally you can fire a skillshot through the wall and kill just the big wraith, but if not then try to leave at least one wraith alive so it won't respawn, and the enemy jungler is denied gold and experience. You can also ask your jungler if he minds letting you take his wraith camp, depending on how well he's doing.
A big part of mid is also ganking other lanes, and if their tower is gone it's generally acceptable to come down to gank, and then push the lane, so you can grab some CS that way.
Most mid champs will also have at least one AOE nuke or demi-nuke, which should let you take several minions at once, so you should be able to ensure you're getting at least the caster minions in your waves.
Getting your jungler and bot lane to come help you grab dragon will also help your entire team out, a single dragon gets your team 975 gold.