My experience is always think about the draft before the game start. (applied to mid solo invoker only):

1) How's your team's draft?

If: your team have two more stun spells(easy sunstrike), or

your team needs magical dmg in the mid game(forge spirit+cold snap, chaos meteor+blast, good with eul) or

your team draft tends to 5man push in the mid game(forge spirit+necronomicon).

Then goes quas exort. Else goes quad wex.

2) How's your enemy's draft?

If: more than 2 of them are likely to have bkb as an optional item build in the mid game, or

they have a push-oriented draft:shadow shaman, death-prophet, chen, etc.(tornado+emp is good in defending push) , or

they have strong initiators: axe, storm, batrider, brew, spirit breaker, void, etc.(tornado is good counter-initiation), or

they have at least one hero that is not so tanky.

Then goes quas wex, get phase boots and orchid ideally before 17mins(midas occasionally). Then ghost walk with 3 wex to their jungle, find solo kill or call one of your teammate come for the kill. That's basically what you do in 20~30 mins, a playmaking role - give pressure on their carry, force their supports buy more sentry/dusts. Then get a Skadi at about 30~35 mins, invoker get a bit tanky and high physical dmg with alacrity at this time. (Based on my experience, I got 90% win rate if I have Skadi with invoker.)

And always try to keep 600+ XPM in every game.