We dedicated Engies are a dying breed! Enemies getting smarter while our poor Sentries are still the same dumb, wall-hacking, aim-botting inanimate machines they once were 7 months ago. But never fear! They key nowadays is to leave a Level 1 or 2 (never 3) in odd positions throughout maps. You'll get maybe 3 or 4 kills, but thats enough to thin out your opponent's team, even if its only for 10 seconds.
What was once a common sight of an Engy wedged between two slabs of metal, its much more effective now if you separate your buildings. Decreases the chance of an enemy "breaking your stupid crap." The only exception is if youre base is coming under heavy attack, where a Dispenser behind you is vital. :O
Oh, one more thing. A Level 3 Sentry only has roughly "200 HP." When you see a Demo lobbing stickies, shoot them away with your pistol! It takes 3 stickies to turn everything, including you, into tiny chunks of flesh and steel.
One final advice. If you see an uber rushing towards you, whether it be a Pyro, Demo or Soldier, get the hell out of there! Their damage output, especially from the Pyro, at close range far exceeds your ability to repair the buildings. Heavies arent so much as a problem if they're foolish enough to fire from mid-range. But up close, or around a corner, just bail. No point in sticking around and dying. Plus it means you'll be able to intercept them with another Sentry down the map.
The best advice though is to watch this Engy video by Daedulus himself

[ame]http://www.youtube.com/watch?v=ZxodkFM8N1U[/ame]