Let them push you to your tower, only close into minions to Q (Buy an initial Doran's shield to be able to soak the initial damage before they push).
Call for a gank, if that isn't possible, keep only closing in for Q last hits
If you eventually run out of health I advise getting the teleport summoner spell, the winner here isn't the harasser, but the farmer (Unless you get killed).
Don't facecheck bushes, wait till your enemy reveals himself. (The garen thing you mentioned)
Do not overextend if they have a jungler or mia.
Doran's armor and regen should be enough to keep last hitting AND ONLY last hitting, if your not last hitting, you should be chilling behind your minions out of enemy attack range, but in minion xp range.