I've always focused on cooldown and mana with Howitzer, but with the advent of Monolith I've made some big changes and overall this improved my ability to lane, I find this is especially suited for mid-laning once it's up and running.
The build now focuses on out lasting with more mana to spend, dishing out pain with damage pen and increase to damage overall, with a slight boost to attack speed and some cooldown reduction. When properly set up you should be in a position to hold off all but the most intense lane pushes.
The abilities are really well explained and laid out in @Deevox guide Me Go Face so all I'll say is □ is going to pretty much replace your R2 basic once you've got your mana regen up to speed.
○ is great for throwing lingering enemies into the range of your tower or pull them forward so you can Ult behind them to either zone off their escape or force them to walk through it.
R2 is another good setup for from Ult, or to slow down your opponent for any other reason. It's not as important, but it is handy. Slow enemies under a tower die faster.
Abilities upgrade priority
Q > E > R > RMB
CXP is incredibly important here to stay ahead of your opposition. So hopefully your jungler is pulling their weight. I'll usually start with Amp Crystal or Solaris Reactor as my starter card. The main aim is to have enough mana to keep poking enemies and pushing them away without needing to return to base. If it's just one on one don't be afraid to chase a little, but I'll normally stay within spitting distance of my tower. Impact Hammer is a great second card for a sudden massive damage boost
Mind Flow is going to be key to this build once you have a decent mana pool/regen. As your now hitting every 7 seconds as opposed to every 8.
You should have a decent mana pool now thanks to Mindflow, Amp Crystal and Solaris Reactor, with a Magus Ward perhaps thrown in for measure, and an increase crit chance with Impact Hammer.
Now is about improving your damage and survivability.
To that end, as Mindflow's 10% Cooldown is a unique passive and doesn't stack, it's time to drop it in favour of Chrono-vore increase mana regen and attack speed significantly whilst retaining that cooldown bonus.
By now you're either pushing enemy lanes, or desperately holding your own so your team can turn the tide.
Make sure you've got some basic armour (hopefully you're not letting Ranged or Ability damage close enough) so you can stay in the fight Quantum Casing usually does this job for me.
With a solid crit chance of around 32% stacks of mana or mana regen, a cooldown reduction and damage boosts to the high Heavens you should be able to bring the pain quiet readily to your enemies, enough to either turn a team fight, or cover a retreat.
Card Build under construction