Good locations for rocket jumping:
Often overlooked or unused is the ability to quite easily jump from the ground floor of the enemy’s base to the top, allowing the ambushing of unsuspecting snipers and engineers. Either jump onto the enemy balcony from out the front, or go up through the hole in the grating just inside the entrance. You can also rocket jump from the front of the balcony over (or onto) the metal fences put up on either side for a surprise attack.
This little ledge at the front of the enemy’s base makes a nice vantage point for a number of rocket/sticky bomb kills without being seen. However the folks you kill will see you in their post-mortem freeze frame, so be prepared for their vengeance.
If you fall into the water by accident, you can go to the corner with the raised ground and simply rocket jump back out. Works best if there’s a medic waiting at the top.
Good locations for sentry guns:
Placing a sentry guns in the doorways to the basement prevents all enemies (especially spies and scouts) from getting down there without first destroying the gun, while allowing your teammates to pass through freely.
If your team is running short of engineers, it can be a good idea to place your sentry gun in such a position that it covers both doorways to the basement. It is more exposed this way, but at least it does the job.
If defending the intelligence room, a position that makes life difficult for spies is on the desk just behind the intelligence. To place an electrosapper, the spy is virtually forced to pick up the intelligence, removing his disguise and leaving him as a corpse. Plus it has good sight of both entrances.
Placing sentry guns underwater, contrary to everything you’ve learnt about electronics and fluid dynamics, allows them to make short work of enemies that jump or fall into the water, especially those with low health attempting to extinguish themselves. Note: sentry guns underwater can only shoot underwater targets, and sentry guns above the water cannot shoot underwater targets.