Farm and harass with Q. It gives sustain. Use W to win trades. Use E to end trades after you win them so they can't turn it around.
Shen, to be honest, is a teamwork orientated champion, and cannot treat top lane like an island. In lower tiers of play, top laners just farm, farm, farm and never leave their lane. Even had a Darius go "Can't carry this team" because he as a severe phobia of missing one cs while his opponent led 5v4 teamfights that we just couldn't avoid. Poor guy. I think he has nightmares of missing one minion and wets the bed.
Anyway, jokes aside, Shen's ultimate is a great teamfighting/helping tool that cuts out the time it takes to leave lane to roam. Just ult whoever needs help, turn the tides of the battle, and when you're done just recall, get some items, and teleport back to your tower to farm for the next 2-3 minutes until your ult's up. Rinse, wash, repeat.
Also, make sure to use your Q on dragon and such. That way, your team's damage will heal them, and it makes it easier to contest objectives if your teammates aren't being destroyed by dragon or Baron.
And if you're against Fiora, build thornmail. She will out-cs you early, but if you don't give her a kill you can almost literally make her kill herself with that taunt.