1. put up a sentry first because there is usually enough metal near by that you can build a dispenser with once the sentry is set up
2. ducking behind your sentry allows you to stay somewhat safe from flying bullets, having your dispenser near you helps as well
3. teles can also help you know when someone is coming. Like a spy, they will destroy it or have to jump over it
4. try not to put a sentry out in plain sight. It is likely to be shot down from farther away.
5. beware of demo men!!! lol they like to sticky your stuff!

6. and yes spies too like to stab you from behind as engy so make note of that, watch for them being disguised as a team member.
7. and know that if you are the only engy, you CAN DO IT!!! lol because sometimes it is difficult if you are the only one defending the intel.