I would really recommend watching GSL code S if you aren't already. About 50% fo the games are the absolute top level of TvT. TvT you only want to out expand your opponent when you have an advantage so that you won't just die to a hard push. Basically from here it is a war of attrition where positioning and getting the most out of your units is of the utmost importance.
Some important points:
1) Tank/marine/viking/medivac is the simplest and easiest combination to use where you want to hold any watchtowers and have either a contain on your opponent or atleast on your half of the map. with tanks spread out in lines and marines interspersed with them you should be able to hold pushes cost efficiently.
2) As soon as you reach the midgame (and often before then) make sure you are ready for drops with a squad of marines in your main and add turrets all over in the lategame.
Vikings patrolling likely drop/banshee harass locations will help stop these damaging you.
3) Watchtowers/vikings/scans are essential to give your tanks vision. Get used to trying to hold good positions.
4) Practice your spreading so you can use marines to overwhelm tank lines efficiently wherever they are spread to thin.
5) Get comfortable in long drawn-out games where constant macro and preparation for attacks are essential.
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