1. Choose an area a bit out of the way. Try something not too close to the enterance (though minisentries seem to work wonders there), and with more than enough cover from the bots.
2. Put the exit portal halfway between bot enterance and player enterance, try to keep in a room that is out of the bots' way.
3. Know your neighbors. I'd reccomend one engineer and one pyro. Keep them close, and make sure you all occasionally spycheck each other - spybots are a huge pain.
4. Be ready to move. Sentry busters, spybots sapping fellow engi's sentry, or battlefront changes require quick, well thought moves. You decide - take the dispenser, or the sentry. If it's a level 3 dispenser, there is no choice - TAKE IT.
5. Make sure you're near a health pack. Okay, maybe this should be higher up, but I'm making this up as I go along, thinking of my past experiences. Health packs are just plain good, especially since they provide more, faster health than the dispenser.
6. Keep your friends close, and your teleporters closer. But keep it from interfering with anything - with 2 way teles, you don't want to go back to spawn when just moving around. However, returning to spawn will definitely help if the battlefront moves there - moving your sentry and teleporter back to the bomb hatch will be a huge help.
7. During breaks, moving equipment will instantly raise to level 3, not counting minisentries. RIP Level 3 minisentry, we loved you.
8. Frontier Justice + bonus minisentry near botspawn makes for a very dangerous engineer. If you're heading out to fight the bots yourself, the revenge crits+an extended mag+better ROF will be a /HUGE/ help. Just be sure to improve your reloading, too.
9. Just a tank? Set your sentries, and follow the tank like anybody else. Place sentries to cover the most area (but not be hit by any other bots, if there are any), and enjoy. If a bot destroys your sentry, Frontier Justice will be even better - revenge crits KICK ASS.
10. Don't die. Seriously. Don't try running out into the battlefield unless you're very confident and have a loyal medic (preferably with an Ubercharge and lack of intelligence to realize that ubering an engie isn't the best idea).

Hope this helps become a kickass engi on mvm, did this while half asleep.