Marine/Tank/Medivac is the most cost effective unit composition in TvT, unless they are going mech (Tank /Hellion/Viking). In that case, you need to add Marauders to your army as Marauder/Tank is really good against mech. So, once your macro is all set up, and you have stim and combat shields, a couple of Medivacs and a handful of Tanks, it's time to go out and put pressure on your opponent.
The key to Marine/Tank is to get sieged up in really good positions and force your opponent to attack in to your already sieged Tanks. Using drops (just 8 Marines in a Medivac) is a really good way to harass your opponent and it also forces them to pull their army back to deal with it which gives you a good opportunity to push forward with your main army and siege in a good location.
You have to try to be cost-effective with your engagements at all times. It is better to just play safe than to run into a situation where you aren't sure if you will come out ahead.
Upgrades are also super important in TvT. If you upgrade +1 vehicle weapons then your Tanks will kill enemy Tanks in three hits instead of four, which is a huge advantage if your opponent does not have +1. Marine upgrades are huge too. If you do an eight Marine drop in their main and you have 1-1 upgrades and your opponent is 0-0, he will need to send a lot more Marines to deal with it, opening up different places for you to attack.
I really like using nukes to push my opponent back when this happens. You can nuke their Tanks, just make sure that you are far enough back that they cannot hit your Ghost, and it will force your opponent to unsiege and back up. When they back up, you need to advance your Tanks forward, and continue doing so until you can find a hole in their defense or push one of their expansions.
If both players are evenly matched, it can end up being a split map hour long game pretty easily. If that happens, don't worry about taking all your bases as fast as possible. Just make sure that you have at least three bases running, and secure your side of the map from drops with turret rings and sensor towers everywhere. There is a point in TvT where you should realize that you will never be able to kill each other if you stay on ground units. You have to realize this point in time before your opponent and get ready for an air war ( which is not necessarily a bad thing, I for one enjoy those huge sky terran fights
The next phase of the game turns into BC /Viking/Raven/Thor. Don't try and get there too fast though, you need to have enough money saved up so that if your opponent decides to all out attack you and trade armies, that you have enough money in the bank to re-make Marines/Tanks while teching to BCs.
If you see your opponent teching to BCs too fast, usually you can kill them before they get enough BCs. If you have 3-3 Marines and they have 0-0 or 1-1 upgrades on their BCs then the Marines will absolutely destroy them. BCs are only good late game if they have 2-2 or 3-3 upgrades. Once BC's get 3-3 then the Marines barely tickle the BCs and the Marines become useless.
You want a balanced army of Vikings and BCs. You only need enough BCs to kill your opponent's ground forces. Vikings are the best air to air unit in TvT. I would say you only need like six to eight BCs and the rest Vikings. Also, two to four Ravens are really useful if you can spare the extra gas. When the fight starts, send the Ravens in to suicide Seeker Missile your opponent's Vikings.
TvT is fun sometimes, but it can also be really frustrating at other times. If you have a solid gameplan that accounts for everything that your opponent can throw at you, and you practice it and work hard at it then you will find that TvT becomes a lot more fun and easier.